We are remodeling our bathroom. We are putting in a larger tub/shower combo and it might be tight with existing toilet location. We have a crawl space under bathroom so, I wasn't sure how hard or costly this might be.

When a toilet is moved... It's all about the direction of the joists. If you can simply slide it over 8" (or however far you need to) without the interference of the said joists... simple. However, when it comes to crossing over joist lines... it's a bit more pricy , due to the labour involved. Still it's possible... shouldn't be that costly. After all, you still need to take up a sizable chunk of the flooring to do this procedure.
All the best
Robair