If you want to rip out old tile and tub and replace everything (toilet, lighting, floor, vanity, etc.) and you want to hire someone, I would budget at least 6 to 8k. It really depends on how much demo there is of your old tile and how hard it is to remove it.
If you want to, say, reglaze the tub and tile and keep everything else (just paint the walls and vanity, keep the toilet, do some of the work yourself), it could be much less.
When I did the minor renos on 3 bathrooms (paint, medicine cabinet and lighting upgrade, floor, reglaze tub and tile) it was about $2500 or 3k for each bathroom. But I kept the original tub and tile and just reglazed them white.
for the bathroom where I plan to take everything out, and just go with home depot/lowe's replacements, i'm budgeting about $8k. don't have a contractor yet though.