Cal,
I'm currently in the process of a re upholstering right now. I have a 92 Itasca Spirit. If your foam is good save it, that's a huge cost. The following is a cost saving list.
1.Save your foam, if they say its "stained" it can be wrapped in plastic.This is a HUGE profit for them to replace.
2.Offer to remove the pieces yourself-saves on labor cost.
3.Buy your own material,can be found at any discount fabric store.
4.Work on a cash basis.
I have saved an incredible amount of money by doing these steps, which I might add was suggested to me by a very honest upholster.
The answer to your question about how much? My Itasca took 20 yards at 12.50 a yard. Make sure you buy quality material from USA and not China. American made is a little more expensive, but well worth it.The guy i'm working with in Roseville, Ca told me to put a swatch of material from both country's in a closed mason jar for 24 hrs then smell, WHOA, what a difference.
My job includes bed over cab , seat covers for bucket seats,couch,dinette cushions and the valances came to 600.00 now keep in mind this is labor cost only with me buying the material and removing the items.
Hopes this helps.
Cal