My mix is about 60% of a good peat-perlite based potting soil (like Espoma organic potting mix) and 40% compost, plus appropriate proportions of worm castings, Espoma Tomato Tone, and some pine bark mini nuggets if I have them. I realize that this can get pricey, but you could re-use peat based potting soil a few times if you re-introduce the amendments, I think you'd be okay with mix of about 50% reused soil, 40-45% compost, and a mix of worm castings, pine bark minis and Espoma Tomato or Garden Tone.
If trying to save money, I'd bet that local tree companies might be able to take care of getting you some wood chips to mix in as well in small amounts per batch of soil. With the wood chips, just be aware that they'll draw a small amount of nitrogen from the soil as they decompose, plus you'd want to screen them, and of course you want them to be from a clean reputable source.