There are a ton of variables which will determine the cost.
Will it be on a new truck chassis or used. If it is new there will be a few modifications to get it ready. If it is used there will probably be more. What size motorhome, what options? When your using your own truck you purchased, there will be items that are open ended which can't be determined until they dig in and really see what you have. My cab was one of those, they gave me an idea, but would not know until it was done. Which Dealer will use, will you go with a completely custom truck or one on a dealer lot.
I would say you have the best shot at wheeling and dealing on a unit sitting on a dealers lot which is ready to go. I can tell you these conversion companies do not have nearly the margins into these units as do the traditional motorhome manufacturers. They build one at a time, per customer or dealer specs. No production lines and less of a oportunity to benefit on mass produced items and quantity discounts on materials used.
As for my coach, I will have somewhere around $130,000 plus wrapped up into it when all is said and done.(This is a guess, I really need to sit down and calculate everything up) I have seen dealers moving rigs on brand new FL112 chassis for $140,000. But for me it was easier to make this conversion happen by paying for it in smaller chucks than all in one shot. Meaning buying the truck, then parts for it as I could afford it, doing as much of that work as I could, then financing the motorhome section.
For the most part most of the conversion companies like Show Hauler are priced in the same range as the bigger gas motorhomes 36 + range and the lower end diesel pusher market.
2003 28' Show Hauler Motorhome on a 1995 FL 120 completely refurbished chassis. Currently in production - scheduled completion 12/10/02 www.showhauler.com