This looks good and is cheap enough to make. I don't think I will anytime soon, but the fact that it exists gives me some pause to consider it.
I've used regular white vinegar to remove heavy rust on steel with good results.
Not quick, but pretty gentle.
After a few days of soaking, I pressure wash it. The oxidation is still attached, but the pressure washer strips it off. It will flash rust, and gets a layer of light deposits on it when dry. I will usually media blast the part after drying. Some type of liquid metal prep is required if you are doing the inside of a steel gas tank.
As far as wanting the most economical solution I find nothing beats Rust911 in the 1 gallon concentrated jug. It makes 16 gallons and at that dilution level it is as effective as Evaporust. At $80 for the gallon that breaks down to around $5 a gallon. For me it works to have plenty of solution in shop and not have to continually buy single gallon jugs of other brands.
I think I shared this earlier in another thread.
Another simple but effective homemade rust remover solution -