I don't think I've ever really charged for rust stains unless it was in the form of a little higher cents per sq. ft. rate. I've done some pretty bad ones,but normally I don't have to spend a lot of time or money to get rust stains out. It's normally one of the easier stains to remove if you have a 1/2 way decent rust remover. I've used Chemspec too for many years. Since I've been using it a quart has nearly doubled in price. I decided to pick up some Matrix Rust Remover @ 8.95 a qt. and give it another go. The last time I used it was several years ago and it was an abysmal failure. They evidently have improved it. So far it's worked every single time. I'm a bit afraid of Whink even though I know there is little chance I'll get that stuff on me. It is a dangerous acid and I just don't feel the need for it personally. Maybe I've read one too many msds sheets so I'm a little over paranoid.

I don't normally charge anything for treating a few rust stains. Not knocking anyone who does.
There have been many times I haven't charged extra for certain stain removal that I know I should have. I got into a job about 2 years ago where I was doing a pretty large 2 story home and the stains were numerous and VARIOUS! I think I can say with pretty much certainty most cleaners would not have went to the extremes I did to remove almost every stain in that house and trust me, there were some difficult ones. I easily spent double time cleaning that place as a direct result of the stains. Name a stain and it was probably in there accept I don't think there was any mustard or turmeric.

Food dyes, numerous food stains and drink stains, lots of UFO stains. (Unidentifiable Freaking Objects). There there was the huge stain from a printer cartridge that somehow exploded they told me. Made a very large stain in a computer room or office. I knew going into it that this place was going to be trouble. It did not disappoint cause it was a LOT of trouble!!! I let myself get taken on that job big time!!!!