Don't get too down, I think you may be able to fix your situation fairly easily if the border outside the south edge of your greenhouse gets hit with a lot of sun. (reflectors won't do you any good if that area is shaded)
You've already built a fine greenhouse, all you need is more light energy hitting the plants. Reflectors are easier to build/buy than you might think. Last year I zip-tied together a bunch of blank white signs that I had leftover from a cancer fundraiser that I organized, so it didn't cost me any extra money, but they can be economically made from many different things even if you have to buy the materials.
One thought would be to get some of those cheap 4' x 8' styrofoam insulation panels (from Home Depot or some such place) that have aluminum foil on one side. If you got 5 of those and laid them side by side on the south side of your greenhouse (suitably held down, perhaps with concrete blocks like I used to hold down my panels), you'd end up with a big reflector that would measure 20' x 8'. That would bounce a ton of extra sunlight into your greenhouse, and would probably give you the light energy you need for strong growth.
4' x 8' panels of plywood (which you could paint bright white and hold down with concrete blocks or some other method) would be more robust, especially if you screw 2' x 4' boards all along the border of the big 20' x 8' reflector created by joining the plywood panels.
If you do something along these lines, burying the edges of the reflector a bit will help prevent wind from getting under it (I buried my edges and used a bunch of concrete blocks). And make sure you have a good pair of sunglasses
I bet you could come up with a good solution for a big robust reflector grid for $100 or less. It stinks to have to shell out more money, but on the bright side, a reflector grid will save you some money by lowering your heating costs (thanks to all that extra light energy).