I see it as: you bring us your car, we'll find an engine on ebay and install it. I can see $10k being fair for that. if you want other services such as rebuilding the engine etc, then just add on to the base cost.

I had my 170 SLK already, bought a 01 CLK55 with title issues for $1000 and then a Crossfire 6-speed with some other bits for another $1000. granted, mine is powered by standalone but I still have ESP and cruise. so I paid less than 4K total for EVERYTHING and have an SLK"55" with a 6 speed that has 160k. If I could do the same thing and charge others 10K, I would be making some really good profit.