Amusingly, I play in wedding bands (we also work other sorts of functions such as corporate events). The bandleaders certainly have a la carte pricing. Do you want a singer? Extra hours for setup? Emcee service? Travel to a distant location?

The venue for the event is a good indicator of how much money the client has.

The musicians themselves charge different prices. They expect to get paid more for a higher budget event.

If client A recommends the band to client B, then B will usually know what A paid. That's OK, because a referral gig is usually easier to book, and worth keeping the spigot flowing by sticking with the same price.