Pete,
depends on what kit it is as to the time to build it, other things that can change the amount of time are equipment, if it's all manufacture suggested items, they will probably drop right in, if it is different engines, servo's, gear etc, there may be modifications needed while building.
When I bought my Super Bandit kit I asked a couple builders how much to build the jet ready for paint, all equipment installed and then removed so I could just drop it all back in. I was quoted $3000 to $3600. I built it myself in the end, but I can see it being that much easy. I have over 150 hours in my super bandit and that is the all composite version built with all recommended equipment.