We built our greenhouse out of a machine shed (so it has a tin roof), using scrap glass from all over. We used two bays of the 48 foot long shed, and I was thinking about using the north bay for raising chickens (a la Solviva). But now I'm thinking about turning it into a small "cabin in a shed" (a shabin). We'll see this summer what happens.
I got a couple of solar water heaters that I've built into the south facing wall. We're far enough north that in the winter the sun isn't really overhead, anyway, so I didn't mind having a tin roof. I haven't hooked them up yet, but am thinking about heating large tubs of water inside, as thermal mass, running the piping through some car radiators.
This ran into a few thousand dollars (mostly for the Rocket Mass Heater inside, and the OSB for the walls, paint, etc.).
We had tomatoes until November 9th, I believe, which I thought was pretty good. If we insulate the ceiling, we'll do better -- so I'm hoping to do that this year (especially if I can find some scrap foam insulation). Right now there's just house wrap between the greenhouse and the tin roof.
I got all the pink insulation from a fellow who had bagged it in black garbage bags for a project of his own, which never materialized.
Have fun, whatever you're doing!