I’ve been to 3 of these so can only comment on them (and in fact haven’t been to Tanzania since I was kid so just 2!). Your budget will go a lot further if you book things yourself rather than go with a firm - I appreciate though that this might be a bit scary if you’ve not done this before. It just might make the difference though between being able to afford to go somewhere or not!
We went to Sri Lanka last April - the monsoon season was just starting up. May is a monsoon month and the South West and South Coast (unless you don’t mind rain) are best avoided. The Cultural Triangle, Highlands and East Coast beaches are fine though. It’s a really beautiful country and the people are quite exceptionally friendly. Travel takes a while anywhere on roads so expect long journeys. We didn’t do them but there are some apparently exceptional train journeys through the countryside. I planned our trip myself and hired a transport firm and booked our hotels (either 4 or 5 star)through booking.com. It was a pretty cheap holiday all in without compromising on quality. If you aren’t that confident you may want to either talk to a travel agent or go through a tour firm but if you want some tips on how to do it yourself, happy to help!
Unlike Sri Lanka, there isn’t a great deal on the historical and cultural front to see in Costa Rica. No old ruined Mayan cities etc. It does have an interesting history and rather unique politics in the region which is interesting to know about (if that interests you, appreciate it might not!). San Jose is a bit of a hole. Where it does win is on the wildlife and spectacular scenery front - it’s a genuinely beautiful country. We did San Jose/Arenal/Manuel Antonio - again booked by me using local transport. If you like an active holiday then this is the place for you, whether it’s hiking, canoeing down rapids etc. That’s all easy to organise but again, if you don’t feel confident then look for outside help.
As a quick comparison, both are stunning visually. Sri Lanka has more to see on the history front. It also costs less to eat out and the food is a lot nicer! Both have beautiful beaches (although we couldn’t swim on our beach on the South Coast in SL). Costa Rica offers more on the adventure holiday front and getting from A to B takes less time sitting in a car! Both have big National Parks and a lot of wildlife so go and see wild elephants in SL or sloths in CR!
Again, if you want any help, happy too.