Where are you based? That matters a lot because 1) ticket prices and 2) your passport.
I can start helping you out with this link, but with that info I can help you better! https://steemit.com/travel/@borderedge/how-to-find-the-cheapest-flights-from-anywhere
I'm based in the U.S., basically this will be my first time leaving the states
According to Skyscanner prices for Costa Rica (San José) are about 20 euros cheaper (183 vs 204) and the cheapest ones are both direct flights from Miami. So if you come from the south it doesn't make much of a difference between the two, if you're based elsewhere it depends, even though Costa Rica seems slightly cheaper. I've never been to either country but I guess Costa Rica is a good start.
Otherwise I just found this http://www.secretflying.com/posts/summer-miami-belize-201-roundtrip/
I appreciate the input! I think Costa Rica will be my first foreign trip now!