You offered to do the work and now you want to get paid for it? Not cool in my books. Unless you were actually out money putting up posters and such, I would say no you are not entitled to more pay.
Now if you had called the replacement band and just said, will you do it for $50 and pocket the difference, while shady is not nearly as offensive, since they agreed to take the $50.
Time is money friend. I put in between 6-8 hours of work between contacting bands, screening responses to ads, mocking up posters for approval, etc. Thats beyond weekly rehearsals and the time and effort it took to GET this gig in the first place.
But I see what you are saying. Everyone is on the gig "on spec" essentially, everyone has a new CD out and is trying to move units. I'll ask my singer who is working the bar directly. I'd almost be cool making the guarantee our take, then letting the other bands split the bar take. It all goes back into the band fund in any case, paying back for recording expenses and soon for tshirt printing, etc. We're not paying any bills with gig money