coaster brake ftl
 

I have a coaster brake on my beachcruiser, which is singlespeed. and I want to get a freewheel on the back and remove the coaster brake. essentially get a new hub. About how much would that cost to install and purchase? and for the rear hub, could I just use a fixed/freewheel hub?

yes, you could get a new hub but it would probably be easier/cheaper to go scrounge around for an old 26" wheel (or whatever it is) from an old bike and slap it on there.
but why would you ever want to do this? wouldn't it be more trouble to have to get brakes for an old ****ty beach cruiser?

Assuming you find an inexpensive SS rear wheel (cheap 5sp 26" wheels exist, cheap SS wheels are harder to find), a couple cheap BMX sidepulls, and a freewheel, you're looking to spend at least $50.

If you have access to a bike junkyard, this project may be doable. But the lowly coaster brake truly is the solution to lots of problems. If you must have a cheap ss freewheel bike, go to walmart, get the cheapest MTB, park it on one gear and ride it.