What is the best/ cheapest way to get to Costa Rica from Belize?

I am wondering the same thing. I'm going from Belize to Costa Rica in the beginning of January. Flights are around $330-370 to go from BZE airport in Belize to San Jose, Costa Rica.
But if anyone knows of buses that make the trip and are safe I would love to know. I don't speak spanish and my travel buddy will be leaving Belize before me, so an easy to find bus station would be even better! Any advice would be appreciated!

They are not close together. You could take a series of buses and overnight lodgings in Guatemala, Honduras, and Nicaragua in order to get there, but by the time that's all said and done you probably will not have saved much money.
Tica Bus runs from Guatemala City to Costa Rica for $82 one way. Of course, you have to book your lodging in El Salvador and Nicaragua as well as paying for food. That probably brings you near $200 out of pocket plus two straight days spent inside a bus. Plus whatever it costs in time and money to get to Guatemala City from Belize.