Is the cost based on an already built-out facility? Or are you working with a shell?
As a gym owner myself who has gone through numerous constructions of my own facilities (they were all shells) at least one shower was a must for me.
If you are working with a shell, it may behoove you to place the shower in now because if your members later request it after the build-out, restructuring the plumbing later can be a big pain and really expensive.
I do concur with Ron that retention should be consideration.
I hope this information helps and I wish you the best.
Lawrence with Gym Insight