Hey Tomm,
the taxis aren't too bad, about US$25 from the airport to the marine terminal. Taxis on-island are reasonable, since the distances are short. They'll want to charge you a few extra bucks, since there's a group of you. Ask up front. Taking the hop in one of
Maya Airways or
Tropic Air's light aircraft will cost you about US$55 each way from the international airport to San Pedro US$40 from Corozal to San Pedro. With four of you
backpacking, you're probably looking to save a few bucks where you can. The water taxi from Belize City costs about US$10, according to
this page. I thought it was more than that. The Thunderbolt used to have a webpage with pricing, but it seems to have gone off the air. You can find information regarding the fares for many of the transportation companies in belize
here.
  As far as nightlife, to each his own. The island nightlife is laid back. It's okay, but nothing to write home about.
    Dan