Once your project is complete it will be well worth it. All my favorite tanks over the years have ways been in the wall.
1) determine if the wall you want to mount the tank in is "load bearing". A load bearing wall supports a floor, ceiling and/or roof above it. Generally speaking, if the wall runs perpindicular to the floor or ceiling joists it's load bearing. If the wall runs parallel to what's above chances are it's not load bearing.
2) if the wall is not load bearing life is alot easier.
3) if the wall is load bearing the load from the removed studs must be spread to jack/king studs. I'd recommend hiring a professional unless you are a serious DIYer.
4) be aware of wiring hidden in the wall prior to making any cuts into the wall.
How much $s? Depends, DIY for a couple of hundred, hire a pro 2-3 times that.
I design and build houses so if you have any Qs PM me and I'll do my best to assist.
An in-wall tank is sharp. Good luck.