Also check Orlando - Jet Blue, a very good airline, flys to SJO from there - I always thought it strange that they don't have an option from Ft Lauderdale, b/c they have a presence in that location - worth checking, although as El Silenco points out, Miami has more options to Latin America period.
Charlotte, NC also offers SJO flights (US Air?)
I don't like the bullshit w/ Spirit (wow, what a textbook case on how to conduct your business to have people - your customers - hate you - I always thought that being the cheapest was a bad stragety, b/c there is always someone who will undercut you) but have taken them the last few times - they were less expensive, even with the baggage and seat fees
The best experiences over the years from top to bottom have been w/ TACA out of MIA - but they have not been price competitive ( >$200 vs Spirit) the last few years.