add a tub where there isn't one??

I'm looking at buying an older house (1976) that has just a shower in the downstairs master bath. One option is to replace this with tub/shower unit, but my wife is partial to having them separate. It seems that adding a bathtub to another area of the bathroom would be a real pain with having to run new drain lines and such.
The layout of the bathroom is such that I was thinking maybe the tub could be raise on a step-up to allow running the drain over to the existing shower, without digging holes in the concrete. Could this work, or should I look at having the plumbing redone under the house? (yikes)
Also, anyone have a rough guess on what the plumbing work might cost? The rest I would do myself.
Thanks,
J

Even if you built the tub up on a platform, you still need to get below the slab to connect it to the drain. If I read you correctly, you thought of running the drain into the shower? Hinky, not a good idea.

A clawfoot tub wouldn't require a platform, but most any drop in job does.

Breaking up the slab isn't a big deal unless it is a post-tensioned slab (more common to earthquake areas). If you don't have one of those, then it isn't that big a deal. You could do it, if you wanted if you are reasonably handy, you can rent a demolition hammer. A neater job could be done (in the hole, not the overall mess) with cutting the slab first with a wetsaw. But, that generates a lot of water runoff, which could be a big pain to contain. Doing it dry makes a huge dust cloud.

How much and where you need to break concrete depends entirely on how the drains a layed out under the slab...sometimes, you don't know until you start breaking things up. there are ways to figure it out wihout breaking it up first, though.

Price depends very much on location. For around here a simple tub swap out or install is $550 & up....$800 average, not including finishing the walls. Including breaking up the concrete... I'd guess at $800-1200...with proper tools & equipment I do it in about 13 hours...including new drywall and first coat of drywall mud...ready to finish and paint.

I spoke with a local contractor today who told me something similar to what you both said. Thanks for the info. I guess if I could figure out where all the plumbing is, I wouldn't be totally against doing it myself, but those prices don't seem to bad to have it farmed out.
I didn't really expect running to the shower drain to be a great idea, just thought I'd throw it out there.

J