The simplest way is to find a company that builds greenhouses, write them a check for $50,000 and see what develops
For anything cheaper than that, like s0rcy says, you're going to have to give more inforamtion about what exactly you want (your growing zone, chosen site, purpose of greenhouse, how much of the year you'll use it, $$, etc)
Just so you know going in, a greenhouse that you can grow warm-weather-loving things in through the winter is going to be fairly expensive, both to build and to operate. Someting for just spring and fall use could be simpler and cheaper.
there's a thread a little further down called "do ya wanna build a greenhouse" a lot of diferent gh and cf options have been discussed there it could at least give you some food for thought.
i agree with pat, except i build gh's so just send me the cheque and i'll run right over and hook you up.hehe