Traveling on a budget - college student?

So me and a few friends are looking to travel somewhere over christmas break (sometime between jan 1-22). We want to go somewhere "exotic". We were looking into costa rica, or bahamas, or somewhere interesting besides the states... but the airfare is expensive. We dont mind staying in cheap hostels. We are looking to spend roughly $500 for a week long trip. Any suggestions or ideas would be awesome! Thanks.