You can't measure "properly". You can measure enough to know that you'd be blowing it if you spent 20k.
If spending 300 didn't bring in at least 3 users over a week, that advertising was never going to work. If it did, then you go up an order of magnitude and test again. Never scale more than one order of magnitude at a time.
You really don't know that. It takes time for the algorithms of companies like Facebook to optimise properly. For my company, CPA has come down significantly as we have spent more.
