You can get good deals anytime during the year. I used this website for traveling down to Costa Rica with my family and it proved to be an amazing resource: http://www.naturefamilyadventures.com/index.html. I'd recommend it to anyone!

Well right now.
winter here in the US .
spring

Conventional wisdom says the best season to hit Costa Rica is the December-April dry season -- of course, that also means higher prices and busier hotels, beaches, attractions, etc. Try the shoulder seasons (April to May, late October to mid-December) for fewer crowds, lower hotel rates, and better airfare deals. (Incidentally, isn't that picture you attached of the sea stacks Los Cabos, Mexico, not Costa Rica?)