I've used these Biocide Systems 3220 Room Shocker Odor Eliminator and they work great. Just do not use them around pets. It takes a while before you can enter the room again, but it works and I think it's worth it.
You can also try using a few of these around the house https://www.amazon.com/gp/product/B00B3XUBES
i've only used this once and it was to deodorize a room that was very stuff from having old food and pot smoke in it. But it worked very well for that. Using these and/removing rugs/repainting should do the trick i think. You do however have to let these sit at least 4 hours (we let it go for 24 hours) and you can't be in the room while it's working
I had a younger cousin staying with me when he first started college and he smoked in his bedroom. Really stunk up the room.
https://www.amazon.com/gp/product/B00B3XUBES/
I used that to clear the smell out of the room after i cleaned it up. It worked well. Only downside is you have to keep the room dark and stay out of the room for a few hours/a day, while it works.
As for the price - do you plan on hiring anyone to clean/fix anything? if so try to get a quote on that before you agree to anything. I would also have the house inspected for any bug infestation. People who live like that tend to attract roaches and whatnot.
There is a product called TXC or something like that, starts with a T. You basically scrub or mop the walls with it. Not sure how well it works after paint has been applied over smoke tar. Sorry, I'm on mobile, hard to look up.
Also, this room shocker:
Direct from company: http://www.biocidesystems.com/room-shocker-clo2-odor-eliminator-quick-release-vapor-2?gclid=CKrykOL__cYCFZcYHwodWYIE_Q
Apparently it only works if you totally remove source of odor, then block of all sunlight, and room has to be warmer than 50 degrees.