I agree with others that if the package is for an all-inclusive hotel at a tropical destination, it's likely that most guests will be lounging around the pool with drinks, and sticking to the hotel. If you know the other guests, staying at the hotel with everyone else will be an amazing opportunity for quality time with your friends (where everyone is relaxed and doesn't have to plan anything). If I were you, I would think about how close you are to the couple (and whether you want to remain close in the future), and how you feel about the monetary cost. I would either go, and do the whole package and eat the cost, or decide not to attend and as graciously as possible break the news to the couple (and send a card + gift, if I want to show I value their friendship). If you choose not to attend, I might reference work constraints - the fact that you recently went on an extended vacation might actually be an explanation as to why you can't take yet another vacation. Or perhaps you can reference the timing not working with your company's cycle.
It's worth remembering that weddings are rife with symbolism for everyone involved - which is why if you say 'the cost isn't feasible', the couple might hear 'your wedding isn't a priority - I'll spend on my own vacation, but not your destination wedding'. Remember, a wedding invitation isn't a summons - you don't have to attend, and in fact shouldn't attend if the cost is such that you'll be in a bad mood for the duration of the trip. You don't owe an explanation for why you can't make it, either, but if this is a good friend (who you want to keep) it'll go a good long way to call and reiterate how excited you are for the wedding, how beautiful you're sure it will be, and that you regret it won't be possible for you to attend (but you look forward to planning a trip to see them later, or hearing all about the wedding).
I got married about a year ago, and while I don't have any negative feelings towards anyone who couldn't make it, it is undeniable that I feel warm fuzzy feelings towards everyone who DID attend, who showed their love and support by attending, and who is part of my memory of what was such a tremendous and joyful experience. And there were some friends who I took some time in deciding whether to invite - old coworkers (who I now lived in a different city from) - the symbolism of my inviting them showed that I wanted them in my life, to be my friends in the future. Not saying we wouldn't have stayed friends otherwise (I'm still friends with many people I didn't invite), but it was a pretty significant action on my part, which then lead to them seeking my out when visiting my new city. So just saying that their attendance sent a strong signal that they wanted me in their life in the future, and that we were on the same page regarding that.