I just bought a gutted Alaskan 8' cabover, just this week I spent $3500 on just stuff, no hardware ,labor , or plumbing, or wood.
From memory I purchased: Torklift steps, fastguns, Arb 50qt fridge, SMEV double burner, double SS sink, thetford cassette,suburban 6 gal hot water heater, Atwood 16k btu furnace, 21 gal water tank, shurflo pump.
I guess it really depends on the camper you added it too, when I get done I figure I will have maybe $6500 into it, plus lots of labor. But I will have a real solid camper that is built a whole lot better than the new stuff, for a lot less money.
I also look at the fact that the fridge, Titan jacks( camper came with brand new jacks)fast guns and steps can go with me to my new Alaskan in the future. That's a couple grand thats portable.