T.M.
My husband just built the first box this week!! We haven't tried the mix yet, but my husband says it's important. He has been doing his research on it for months and he said compost and peat moss are easy to come by. It's the vermiculite that's expensive. He said that he'd read several threads that said without the vermiculite the garden suffered. It's what keeps the soil moist. You can find the vermiculite online and that's how we are going to order it. I know it's expensive but once you have it in there, you don't have to add it again.
I wish you luck. We are excited about starting! Don't forget to add marigolds to keep the bugs out!