Last year a friend of mine gifted me quite a bit of pallet stock wood, which is scrap for her business. Yesterday we used it to make some shelves. In an effort to keep plant drainage as efficient as possible, we topped the shelving framework with some sections of wire shelving we had in the garage, and weren't using. It turned out to be a project that required no money. We simply used what had become obsolete for others (including ourselves), and ended up with some nice, sturdy shelves, exactly the right dimension for the greenhouse. I'm very pleased.
We used just a few u-shaped nails to secure the wire shelving to the framework, to pull it all together.
We ended up with four 3' long, and one 3-1/2' long shelves. Next year I can see adding a middle row, as well, but this will be fine for the plantings we started this year.
Have I mentioned how much I love my greenhouse? =)