It probably would be cheaper to build it yourself, rather than buying a kit or ready-made. On the other hand, someone has to actually do the labor. Will that be you?
Decide what you want. What are you going to grow? How big is the GH going to be? How much sun/shade does your build spot get? What kind of construction are you planning? I would start by reading every book I could find about GH construction.
Some of the things you may want to consider are foundation (concrete, wood), frame (wood, metal), windows (plastic, glass), floor (bricks on sand, concrete), are you going to heat it? How will you handle drainage and ventilation? Research the pros and cons to each in your area. Wood frames are cool until they rot away and you are faced with having to replace something that's holding up your greenhouse. But if your area is dry and you aren't raising orchids, the wood could last for 15 or 20 years.
Check with your Planning Department to see what the code is. You may or may not need a permit. You may or may not have to meet certain criteria.
My GH is a heavy duty kit GH. I did have to get a permit because, although you can't access the GH from the house, it is attached to the house. A freestanding GH does not need a permit. If you decide to put a couple chairs and a little table in your GH, it may be considered a 'living space'.
Because I had to get a permit, I also had to put the foundation 18 inches deep. The GH was engineered to withstand 130 MPH winds - that's the building code. On the otherhand, snow load is not an issue.
My GH has withstood winds of 90 MPH and its still standing.
I have friends who built GH without the wind load and their GH's are now residing in Elko - first big wind and the GH headed east. They didn't even find pieces!
There are a lot of GH construction plans and ideas available but make sure it makes sense for your area and what you want to do.