I don't think under any circumstances you will be able to raise 6 juvenile Discus to 6" for less than $800. If you want to show off something nice at the lowest cost, start with a brand new tank and buy a group of adults from a forum sponsor and be good to go from Day 1.
My degree in Economics is pretty useless overall but it helps solve problems such as these:
1 - No way you can grow Discus better and cheaper than someone in Asia.
2 - Opportunity cost of them to raise juvenile Discus is by far the most expensive - especially the 50% daily water changes people seem to be doing at the minimum. Some people when they do 3 feedings daily - this means 3 water changes daily.
I think the exercise you are doing is pretty cool actually - some day I would like to do the grow out juvis for the challenge, fun and sense of accomplishment.
For cheap stands you can go with Titan EZE stands at Dr. Foster. Just metal frames to hold the tanks.
For cheap tanks, Petco does the 1 dollar per gallon tank sale a few times per year. You can pick up a 29 gallon and a 55 gallon. Buy the equipment that would power the 55 gallon but run it in the 29 gallon. Smaller tank will cost less to power, less water to change, more efficient feedings, etc. When the juvis outgrow the 29, move them to the 55. The 29 then works as a breeding/quarantine/hospital tank.
Everyone's heating bill is different but it seems all the US domestic Fish farms are in Florida and not Minnesota. You'd have to find out what your cost per kilowatt hour is from your utilities and then use a device to measure how long and how often your heater runs.
If you have to ask how much acrylic tanks cost compared to glass, you can't afford the acrylic. (I use glass tanks by the way!)
I would love to see others responses but I am pretty sure I am correct that you cannot raise Discus for cheaper than you can buy adults. This might be different for breeders who build an operation for economy scale out but the fact that almost every forum sponsor imports Discus tells you everything about the financial feasibility of raising your own to full size.