The honest answer to what live music costs is that most people have no idea, so they either overpay a booking agency's markup or under-budget and get let down. Prices swing hugely between a solo acoustic act and a ten-piece show band, and without ranges to anchor to, planning a birthday budget is a guessing game.
Then there is the trust problem. You are paying a musician you found online a deposit to hold your date. If you send it to a stranger's payment app and they ghost, your party has a hole in it and your money is gone. A birthday is a fixed date; there is no rescheduling your way out of a no-show.
Middlemen add cost you may not see. Agencies and some marketplaces layer service fees and markups on top of the act's real fee, so the number you pay is not the number the musician gets, and you are paying extra for the privilege of not knowing the difference.

