Cost can be reduced considerably if the greenhouse is a "wing" extending from your home, rather than a "lean-to" type, as the former allows you to buy a less-expensive kit without an end and attach it to an exterior wall.

The first thing to do is consider how large you'd like it to be and where it would go. Do you have the proper light exposure for a greenhouse there?

If that's a "go", looking into zoning regulations and consider what your neighbors would think about the addition.

If you're still in a position to proceed, then you can start looking at structures.