Where should I spend my next dollar?
You have budget to spend. The question is not whether to spend it but where. Here's how to use your actual data to pick the channel that will give you the most back.
The short answer
Where does your next dollar go? Spend it on the channel with the best marginal return, not necessarily the channel with the best average ROI. A channel producing 5x returns at $2,000/month might only produce 2x at $4,000/month. You need to compare current efficiency across channels and look for the one with the most room to scale.
Why your best-performing channel might not be the best place to spend more
Referral is your highest-ROI channel. Infinite return because the ad spend is zero. But you cannot buy more referrals the way you can buy more Google Ads clicks. Referral is great, but it is not scalable in the same way.
Google Ads is producing customers at $300 each. If you double spend from $4,200 to $8,400, will you double your customers? Probably not. You will likely see diminishing returns as you bid on less relevant keywords or target broader audiences. Your cost per customer might jump to $450 or $500.
The right answer is the channel where an additional dollar produces the most incremental revenue. That requires knowing the current efficiency of each channel and estimating how that efficiency changes with more spend. Your ad platforms will not tell you this. You have to figure it out from your own data.
Marginal return: the concept behind smart budget allocation
Budget allocation is about marginal returns, not average returns. Key concepts:
- Marginal CAC. The cost of the next customer from a channel, not the average customer. If your average Google Ads CAC is $300 but the last few customers cost $500 each, your marginal CAC is $500.
- Diminishing returns. Every paid channel eventually hits diminishing returns. The first $1,000 on Google Ads targets your best keywords. The next $1,000 targets your second-best. Each additional dollar buys slightly less.
- Channel saturation. At some spend level, adding more budget produces almost no incremental customers. The channel is saturated for your market.
- Opportunity cost.Every dollar spent on one channel is a dollar not spent on another. The question is not “will this channel produce a return?” but “will this channel produce a betterreturn than the alternative?”
How to determine your best next-dollar channel from ad and accounting data (7 steps)
This requires historical data from at least 3 months. You need spend and customer count by channel for each month. Plan for about 60-90 minutes.
- 1Build a 3-month spend-by-channel table
For each of the last 3 months, record the spend on each channel: Google Ads (from the Campaigns dashboard), Meta Ads (from Ads Manager), and any other paid channels. Also record allocated costs from your P&L in QuickBooks or Xero.
- 2Build a 3-month customer-count-by-channel table
For each month, count new paying customers per channel. Use your CRM (HubSpot “Became a Customer Date” + “Original Source” or Salesforce “Close Date” + “Lead Source”) and verify against accounting.
- 3Calculate CAC per channel per month
Divide each month's channel spend by that month's channel customer count. Now you have 3 data points per channel.
- 4Look for the efficiency trend
For channels where you increased spend month over month, did the CAC go up (diminishing returns) or stay flat (room to scale)? A channel where CAC stayed flat despite 20% more spend has room for more budget.
- 5Estimate the marginal return
For each channel, project what happens if you add 25% more spend. Based on the trend, will CAC stay flat, increase slightly, or increase significantly? The channel with the flattest projected CAC is your best bet.
- 6Check for untapped channels
Are there channels you have not tried that competitors are using? A new channel with no historical data is a gamble, but it also has no diminishing returns yet. Small test budgets can reveal opportunities.
- 7Make the decision
Allocate your next dollar to the channel with the best projected marginal return. If two channels are close, split the budget between them to gather more data.
Total time: 60-90 minutes, assuming you already have monthly channel CAC data. If you do not, add another 45-60 minutes per month of historical data you need to reconstruct.
Why budget allocation decisions rarely use real data
Most businesses allocate their marketing budget based on gut feeling, momentum (keep spending what we spent last month), or whatever the ad platform sales rep recommends. The reason is simple: building the cross-platform data set needed to make a data-driven decision takes hours, and by the time you have the numbers, the month is already underway.
The irony is that budget allocation is one of the highest-leverage decisions a business owner makes. A 10% reallocation from an underperforming channel to a high-performing one can meaningfully change your growth trajectory. But without the data, you are just guessing.
Or see where your next dollar should go, automatically
Bottomline tracks spend vs. customer acquisition across all channels over time. It calculates current and marginal efficiency for each channel and tells you where additional budget will have the most impact.
The recommendation is grounded in data: Google Ads CAC has stayed flat despite increased spend (room to grow). Content/SEO has an improving trend and the lowest CAC. Meta Ads is getting more expensive with each additional dollar. Instead of splitting budget evenly, you can concentrate on the channels that are still efficient.