well, you can do some calculations...
1. check conversion rate.
2. check fb's guesstimate/reach/clicks for that audience you want with 500$ budget.
3. calc your profits based on that:
if fb guessimates 2000 clicks for that audience with 500$ budget, and your offer converts at 1%, it means that 500$ will bring you 20 sales...
if every sale is 10$ like you said, then you made 200... which is a bad deal because you invested 500.
if your offer converts at 5%, then you made 1000, which is great.
play with the numbers.
and as others said, start with low budget then analyse and expand...