it might be to late, but here you go.
Verizon offers a program to rent a phone for 20$ for 30 days if you travel. Make sure you get one that's unlockable (sim card required). I got a blackberry storm from them the other day. Way cheaper to get a prepaid sim in CR. here's all the info:
http://ticotimes.com/costa-rica/cell-phone-service-sim