I haven't partially filled a tank in years, but when I did, the price was per gallon (or pound, I don't remember). Last time when I had my 30# tank filled, it was bone dry, and I think I paid like $25 out the door? Something like that.
I agree with Seann though, if you're just topping off a tank, you should pay for what you take. We have LOTS of propane vendors to choose from, all in close proximity, so if one of them wanted to charge me for a whole tank when I just needed a few pounds to top off, I would just tell them I'm trotting down the street to one of their competitors and will not be back to fill any of the other 4-6 tanks I have.
My guess is many of them would politely wave to me as I left, we have a couple large commercial outfits who are likely more inconvenienced my a small-timer like me anyway.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!