Best target goal? Conversions.

The ability to target these the strong suit of Facebook Ads in my opinion. You can't pay rent with publicity.

Conversions can be as straightforward as purchases if you're selling a product, something like lead capture if you're selling something over a webinar or with a sales team, or even something top of funnel like being added to an email list.

As long as you have a bit of historic data (go with a break even analysis for your first run) you can tie every lead/webinar registration or email sign up to a dollar cost. At that point you're making an investment, not a guess.