Assume it costs more, takes longer, but you get what you want? What kind of percentage premium are we talking here?
Depends on size, layout, difficulty, and overall cost. There are other variables too, but those are the main ones.
Also, Mich is right. you will get a much better deal buying an exiting house right now. If you hire someone to build it, you aren't getting a deal and you can always hire someone to build a paralyzed dick ramp or lift or something.
That said, if you are still getting paid to do nothing, you should totally go test out on the GC license and build it yourself. Should be able to do so in the next 4-6 months and save 10%-20% on the overall.