I pay $250 for a float changeover on my 180. I could shop around and get it for less, but the shop I use does good work, when they say they will do it, with no hassles, so it's woth paying a little extra.
I don't see why it should cost you much more than this. The plane came out of the factory on wheels, right? As long as you have all the parts, it should be a pretty simple process. I don't know the hours of labor, cause my shop charges a flat rate for a changeover, but they always have it done within a few hours of starting. They do a lot of float changes though, so they have the process down.