What is the average cost for a custom tile shower?

That depends on what you are doing.

Take the cost per square foot for your tile and multiply it by the square feet you are covering, Keep in mind that this includes walls and possibly ceilings.

Double the cost of the tile to get the cost of the rest of the materials. You have a shower pan to set, walls to waterproof, and lots of extra things to do.

You also have to add in the cost of the fittings like shower head(s), faucet handles, steam shower, glass doors, etc.

And then there's labor. It takes a lot of skill to do this and it's hard work. It's not usual to spend $3000 in labor alone for a large shower.