I don't know the price per pound formula or rule of thumb. Most in my area are within a quarter or 2 of each other.
You might try the CLA
coin laundry association for a formula.
I don't remember the pounds of each machine, and I know manufacturers are diferent, but its pretty much like this in my area
20lb (double loader) 3.00
35lb triple loader 4.50
45lb maxi loader 5.50-6.00
mega loader $8.50--- yes #$8.50. you got to pump 34 quarters into that thing to turn it on