We just finished building one for our vegetable garden. It was pretty inexpensive and not that difficult to design and build. Much less expensive than buying a kit.

We used 4 mil poly sheeting for the panels rather than the polycarbonate sheets which are much more expensive. 4 mil poly sheeting won't last as long, but on average it should last a year or more. 6 mil poly sheeting can last about 3 years or so.

We designed our greenhouse so that the sheeting can be easily replaced when the time comes.