one reason it is hard to buy peat in large amounts is because, peat comes from peat bogs. they are depleting at a rapid rate along with the wildlife. so peat prices will just go up. growing your own compost/cover crop material is the best way to go. then you can build up your soil and compost at the same time. not that i'm am expert though.
we did it .. right now I only have one bed planted and I actually still have some squares available.
I couldnt do the mels mix the feed store was out of vermiculite and after doing research about it,, it is to aeriate the soil and help with moisture. our soil is airy enough ... so I just added peat and some type of feed.. it looks like dirt.. a bag of mushroom compost .. top soil and planted .. I figure I get what I get and next year through the winter I will mulch, mulch, mulch.
I used Mel's Mix last year with super great success!
Also I found Vermiculite at a local big box home improvement store. It was $8 for a huge bag (I bought 5). You have to look in the insulation section .
Mel's Mix works great. The key is to use really good compost. Well composted horse manure works great if you don't have your own compost heap. Don't buy "compost" in bags at the hardware store. It's not going to work, you gotta use real compost.
If you have a problem getting peat moss or have an environmental objection to using it, the coconut fiber substitute works well, maybe even better. It usually comes in dried "bricks" or blocks that you have to soak and then work it to get it loosened up, but you have similar problems with peat moss if you buy it in compacted bales.
Mel's mix is not cheap. Both vermiculite and peat or "grow coir" are expensive. Be sure to get the coarse grade vermiculite. There is also a fair amount of work involved in mixing the ingredients together to fill you beds, but once you've done it, the mixture works very well. The medium is loose and airy, holds both water and nutrients very well. Since you don't walk on it in the SFG method, it doesn't compact and remains easy to work with using only hand tools or even just your fingers.
You may want to include more than one kind of compost for a variety of nutrients. Chicken manure (not too much, obviously), worm castings, kitchen compost, mushroom compost.
When you harvest a "square" you work in a couple handfuls of compost to replenish it. If you're using good quality compost, you don't really need to add fertilizer. You can spray it with kelp juice or sprinkle in some dried seaweed if you really think you're missing trace minerals. But I think there's gonna be plenty of "traces" in the compost if you are using good stuff. Again, the stuff at the big box store nursery isn't going to work for you here.
I am trying the square foot gardening but am using my own mix which is composted chicken manure/wood shavings and leaf mold and soil... so far I have only used half of one 8ft x 4ft raised bed with my cool weather crops...everything is growing very slowly due to cold wet weather...so the jury is still out...
I have been SFG'ing for 2 years now, after traditional row gardening for 40+ years. I use Mel's mix and have had tremendous success. I think the main problem folks have with poor success is the compost. I was fortunate to find composted cow manure from a local dairy. So many of the bagged composts from the big box stores have lots of wood chips (in my experience anyway).
The initial cost is definitely something to consider, but you can start small & expand. Also, once you are set up, you only have to add compost.