If you want to put bug fixes into the price of the contract, you need to also put the cost of proper QA, test plans, and defining correct behavior, which often means a spec. How do you know the calculation is wrong if you don't have it on a piece of paper?
This all costs money.
Otherwise, you need to be paid. Now, many times you will make a small change for a good customer that takes a trivial amount of time, but this should be priced into the long term value of the customer.
If they insisted on a language/framework/product/etc and that is the root cause of the problem, they will pay for the fix.