Evaporust is expensive, I agree. Now that we're serious about cleaning this block up,
Rust 911 is a similar product and sells a concentrate which is a better value. I have gone through gallons of the stuff. They sell a gallon size concentrate for $75 which makes 16 gallons = $5/gallon. It works just as well as evaporust; its efficacy lasts for about 24 hours after continuous use.
I, too have made the citric acid based solution discussed above. It works very well and is a bit stronger than the off the shelf brands, however it doesn't harm the base metal - only rust. I just completed a 20 gallon run of this solution pumping it through the rusty rocker panels on my New Yorker.
The key to the citric acid is the purchase it at $6.00/lb or less, to equal the cost of the gallon size of Rust 911. The only location I found that sells in bulk is the LA Salt Co. through Etsy where you can purchase 20 lbs of it for $65 = 24 gallons solution: = $2.71/gallon.
https://www.etsy.com/listing/1058913642/1-lb-2-lb-5-lb-10-lb-20-lb-50lb-bulk?variation0=3065383284