Don't listen to people who tell you paid advertising isn't a good idea. If an ad produces the right behaviours (e.g. sign ups to your site) at the right price, it's absolutely worth doing. This tool helps you understand what you can afford to pay for advertising, based on the economics of your business and the conversion rate of your advertising and sales funnel.
CAC = Customer Aquisition Cost. Let's assume here that this is going to be your only CAC for these customers. CAC no more than 1/3 LTV is a standard SaaS rule of thumb. CAC repayment in first 12 months is also a popular rule.
If you want to pay advertisers for each signup.
Here we've assumed that each click on your ad and resulting visit to your site, is from a unique visitor.
Display advertising prices are usually quoted in CPMs - cost per mille aka cost per thousand. Don't ask us why 🙂
Get your ad-spend report as a self-hosted mini-site.