Estimating common home improvement costs?

I’m thinking about buying a home in 6 months or so, and shag carpet seems to be quite popular with older constructions at the lower end of my price range.

Is there any good source for estimating the cost of home improvements like replacing a floor with hardwood, adding a window in the kitchen, or replacing a toilet/tub with a more modern fixture?

I’ve tried looking at the diy site and found estimates for some of the supplies with possible sources, but while I’m handy enough with paint and a caulk gun, this seems like a job for a professional.

Are there any good rules of thumb here? My memories of Maine Cabin Masters or This Old House seem to have them lump all of the improvements together in one budget or not talk money at all.