We are going to St Lucia in March and the only non-stop I can find is out of Philadelphia. I'm thinking of splurging on business class tickets to St Lucia but I'm wondering if its worth it.

St Lucia is served with A319, so you get your basic domestic first class (2-2, 38" pitch) seating on a narrowbody jet. It's not international business service with almost flat seat, 55" pitch, etc.

Flew USAir Business Class from Venice to Philadelphia this past spring. My seat wouldn't hold its position, my wife's headphones had Engish in one ear and Italian in the other and they served microwave pizza to a crowd coming from Italy.
Business class finishes were dated and room was marginally better than a good economy plus.

What is the diffence in costs?

The cost of a flight from Philadelphia to St Lucia round trip coach is about six hundred dollars. The cost of business is double. I was asking because it's the only direct flight to St Lucia I could find and I don't want to spend a lot of my vacation in the airport. But I know that USAir is, well, not doing really well and I was wondering if it was worth it to pay the difference. Apparently not. I like to be pampered but not at any cost and only if I'm really going to be pampered.

US Airways is doing fine.

What do you mean by " I know that USAir is, well, not doing really well" ?