I was charging $14.00 an acre plus fuel with my l2 but I wasn't cleaning the tank out for each different variety of grain. I would charge something for cleaning out the tank each time.
If you are anywhere near my farm in southern Minnesota I would like to hire you to combine the entire place. There would be no waiting for trucks and there is a heated building to park your machine every night. I would also supply all fuel. There are no rocks and most rounds are 1_2 mile long. Email if interested. Tom in MN
Around here, the going rate is $25.00 per acre. It's up to you what to tack on (if anything) for the extra work involved in cleaning the machine between varieties.