If you can't take the trip or don't want to pay the taxes, then PLEASE don't enter. There are people who are so happy to win that trip and it's not nice to enter if you're really not interested.
If the sponsor doesn't say there is a cash equivalent then there probably isn't one and they are under no obligation to give you one.
Say you might love to go to Vegas but have no interest in the boxing match or golf day you don't really have to go, they aren't going to force you but if you'd rather take in a show or something they MAY accommodate you. Just don't count on it.