don't know the difference between party and pro, and the music for them is just an excuse to get loaded. You don't need to put one more second of your time into them, and, as was once said, some of their karma may rub off on you.
I agree with Henry- the pro thing to do is to show up, but do not set up anything until they arrive, and give them one hour to show up. Meantime, do not call them, communicate with them, do anything more for them than play bass. You are just a hire. and let the owner know that you have no responsibility for the band, you are just there to play bass for this one gig. Give them one hour to arrive (from agreed upon set up time). If they do not, leave, and give the bar owner their phone numbers, and apologize for them, and tell him you are not a member of the band.
If they do show up, do nothing more than play your bass. Sit seperate from them on breaks, and if they ask why, tell them you do not want to risk getting dosed. If they act up on stage, back away, or stop playing until they straighten up. If any of their antics threaten you or your gear, garb your stuff and go, and forget about the money.
But you are justified in not even going to the gig. I would do it for the sake of professionalism, but you are dealing with idiots, and it is a choice between sanity and professionalism. I would not let myself be put through that train wreck again- it isn't worth the money or the aggro.