How Much to Spend on Ads for an Online Store: The Margin Formula

How much to spend on ads for an online store? The only honest answer is that the budget does not start from your bank balance or from a popular percentage of sales, but from one order: how much is left after product cost, shipping and fees, and how much of that remainder you can pay to acquire the order. This article explains the formula we use with every store before spending anything, with worked examples for a beauty store and an electronics store, and how it turns into a monthly budget and a MER target.

Why the common budgeting methods fail

Three methods we see in most stores, all leading to the wrong decision:

• “What I can afford”: budget set by cash flow rather than profit, so money goes to losing orders or is cut from winning campaigns.
• “A percentage of sales”: a ratio borrowed from another sector with a completely different margin.
• “What Ads Manager suggests”: the platform suggests a budget that guarantees its spend, not your profit.

The correct method reverses the direction: from the order to the budget, not from the budget to the order.

Step 1: calculate order contribution

Order contribution is what remains from one order before advertising:

Order contribution = selling price − (payment fees + average discount) − product cost − shipping cost

What goes into each line:

• Payment fees: gateway commission, or collection fees on cash on delivery.
• Average discount: the discount actually applied across average orders, not the maximum discount.
• Product cost: purchase price from the supplier including customs and transport to your warehouse.
• Shipping: what you pay the courier minus what the customer pays; free shipping means you pay all of it.
• For cash on delivery: add the cost of returns and cancelled orders spread across successful orders.

This figure is your maximum cost per acquisition (max CPA): pay more than this per order and you lose money on every sale. Cost line details in ecommerce unit economics and contribution margin.

Step 2: target CPA at 70%

We do not work at break-even. Target CPA = 70% of the maximum. The remaining 30% covers unexpected returns, cancelled orders, fixed costs (salaries, apps, subscriptions) and leaves actual profit. In stores with cash on delivery or high return rates we may work at 60%, and for products with high repeat purchase we may accept more than 70% because customer value exceeds the first order.

Step 3: from target CPA to a monthly budget

Monthly budget = target CPA × number of orders required

The order count comes from one of two sources:

• At the start: enough orders to gather judgeable data within two to four weeks, usually dozens rather than a handful, so campaigns exit the learning phase.
• Once stable: the order target from the sales plan.

The budget is not fixed. It rises gradually as long as actual cost per order in store orders stays below target, and stops rising as it approaches it.

Two worked examples with illustrative numbers

Example 1: a beauty and skincare store

Line	Value (SAR)
Selling price (average order)	180
Payment fees and average discount	14
Product cost	70
Shipping paid by the store	18
Order contribution = maximum CPA	78
Target CPA (70%)	55

If the store needs 60 orders in the first month to gather data across Meta and Google, the starting budget is roughly 3,300 SAR a month. If after three weeks the account records an actual cost per order of 45 SAR in Shopify orders, there is room to raise gradually.

Example 2: an electronics and accessories store

Line	Value (SAR)
Selling price (average order)	450
Payment fees and average discount	20
Product cost	340
Shipping paid by the store	25
Order contribution = maximum CPA	65
Target CPA (70%)	45

Note the paradox: the electronics order is far larger (450 versus 180), but contribution is lower (65 versus 78) because product margin is thin. This is why comparing stores by ROAS fails: the electronics store needs a much higher ROAS to be profitable. Here, raising average order value (an accessory with the device, a bundle, a free-shipping threshold) becomes the most important lever for raising the budget you can afford.

Step 4: a MER target for the whole store

Cost per acquisition is measured per channel, but channels overlap: a customer sees a TikTok ad, searches on Google, and buys after a WhatsApp message. Every platform attributes the order to itself, so adding their conversions produces a number larger than the store’s sales. The final judge is MER = net store sales ÷ total ad spend across all platforms.

Calculating the MER target from contribution:

• Contribution ratio = order contribution ÷ selling price. Beauty example: 78 ÷ 180 ≈ 43%. Electronics example: 65 ÷ 450 ≈ 14%.
• Break-even MER = 1 ÷ contribution ratio. Beauty ≈ 2.3x, electronics ≈ 7x.
• Target MER ≈ break-even MER ÷ 0.7. Beauty ≈ 3.3x, electronics ≈ 10x.

This explains why an electronics store needs a MER above 10x to be profitable while a beauty store profits at 3.5x. Read MER vs ROAS for the details.

Splitting the budget across channels and adjusting it

Target CPA is the same for every channel because it is derived from the same order. What differs is how many orders each channel produces at that cost:

Rule	Application
Start with what captures existing demand	Google Shopping and Meta Advantage+ before awareness
A test share for each new channel	Enough budget for dozens of orders within two to four weeks
Move budget gradually	Toward the channel with the lowest cost per order in store orders, in reasonable weekly steps
Never judge 24 hours	Review 7, 14 and 30 days against the attribution window
Protect the winning campaign	No pausing or restructuring without evidence and a ready replacement

Per-channel cost details in Google Ads cost in Saudi Arabia and the Gulf and Instagram and Facebook ads cost in Saudi Arabia and the Gulf.

From our experience

At Sophie’s, a beauty and skincare store with more than 4,000 products, the ad funnel across Meta, Google Shopping and PMax runs against profitability targets defined in advance from order economics, not against platform numbers. At Noor Alhuda, selling in US dollars to diaspora communities on three continents, a 30% MER improvement within two weeks did not come from campaign changes but from a repricing that raised average order value from $122 to $153, so each order could carry its acquisition cost. The biggest budget lever was in the store, not in Ads Manager. See the Noor Alhuda case study.

Conclusion

How much to spend on ads for an online store is a formula, not a guess: order contribution is the maximum CPA, the target is 70% of it, the monthly budget is the target multiplied by the orders required, and the MER target is derived from the contribution ratio and judges all channels together. When contribution is thin, the fix is in price, average order value and shipping before campaigns. If you would like us to calculate these numbers for your store and propose a sensible starting budget, see our paid ads management service or message us on WhatsApp.