The cost is going to depend on where you are and who is available to do the work.
On the surface (especially to a non-bassist) it seems like a simple task: just put the new part on. In reality it takes some knowledge and skills to do it correctly. There's the cost of the actual tailpiece but it may also need some work to fit to your bass, there's the tail piece cord/cable which will probably need adjustment to get the correct afterlengths, possibly a sound post adjustment if the top lost tension and it shifted, and the restringing. All of this takes time, especially the afterlength and soundpost adjustments, which costs money for the specialized skilled labor.
It's a good thing these friends have some paid gigs coming to cover the cost.