Restaurant grease recycling questions

Not sure if any of you have experience with this, but so far for me it seems the buyers are greasing me up.
First buyer paid $.12/# when they paid.
2nd buyer said they would pay $.15/#, but only are paying $.10/#.
Wondering what is a fair price?
& i also am double checking the math & the conversions are confusing.
our grease is classified as yellow grease, & majority says it weighs 7.6 lbs/gallon. True?
Buyer measures grease in tank with a dang tape measure an gives us inches of grease. Now i need to figure out how much an inch of grease in our tanks weigh. hopefully google can locate info on this double walled tank, otherwise it will be pretty hard to do the math.

If you can't get the inside measurements, try this method I've used calculating volumes of tanks for spraying crops. Measure the depth of grease in the tank, pour in a known quantity (say 5 gallons for ease of figuring) and measure again. Divide gallons poured by inches increased even if fractional and you get gallons per inch. Weigh the grease you pour in and divide by the gallons to get lbs per gallon. Then just multiply lbs/gallon x gallons/inch and you have lbs/inch of tank.

Tanks are rectangular & gets wider at the top. Can only get the vertical measurement on the inside.
Pnutfarmer thanks that is a clever method. I dont actually handle the grease, but i may be able to coordinate with the cooks & try & get them to call me when they dump next. Then i could try your method, but it may be too much of a hassle trying to set that up.

Thanx guys, now I have a head ache.

happyscraper - Walk it off, you'll be fine