It really depends on the shoe and what kind of working conditions, stress, and exposure to various elements they are getting. Sometimes it can be due to the way you walk too. You can get most shoes resoled and oftentimes they will fit better than they did originally. A good cobbler can repair separation issues too, where the bottom/sole is detaching. It never hurts to take them in and ask. They can do general maintenance too and clean them up.

I wouldn't say "really expensive", as it is all relative, but to me a good pair of boots is going to run in the ~$150-220 range. You can get some a little cheaper, but I am used to getting boots for construction.