Our favorite questions are monetary ones that are asked not to try to gain money but to make sure that the money a person gets is deserved.
The halachic issue involved is ona’ah (overcharging or underpaying), a Torah prohibition with monetary consequences, including returning the extra money or voiding the transaction when the ona’ah is significant enough. Among the cases where the totality of these laws does not apply are the sale of land[1] and the wages of a worker, which are indirectly compared to land.[2] However, regarding cases where one is paid by the job (as you usually are) and not by time, the Shulchan Aruch[3] rules that the regular laws apply. Actually, even regarding land, the prohibition of overpricing applies, with the difference being in the monetary ramifications once the prohibition has occurred. In any case, since you want to do the right thing, such a loophole would not be fitting.
You imply that one might look at the fairness of pricing on average over the course of the business, in general, as opposed to the appropriateness of each, individual fee. Of course, if a certain price is unfairly high, it doesn’t help that someone else got the better of you a different time. Even if the same person got a good deal in the past because you decided not to demand compensation at the time (thus relinquishing any possible rights), you cannot make up for it by charging too much later. However, the fact that you often undercharge is a relevant factor for the following reason.
Overpricing is forbidden when one goes beyond the accepted range of prices. Several factors help determine what the range is. One of them is the chance that the work will be much greater than average. Consider this example. A taxi driver’s meter usually reads $40 for a trip to midtown Manhattan. If he takes someone at a fixed rate, he has a right to ask for more than the median rate because he sometimes sits in traffic for a long time. His set price of $50 represents the market rate, which takes both opportunities and risks into account. Thus, as long as your estimates are within the market range and your clients agree in advance, you need not worry about fluctuations in either direction and can accept payment as agreed.
You should, though, consider the root of your occasional overestimation of the work. If you find a given paper easier than expected, you may have been concentrating extra well or it is your good fortune that you received a relatively easy paper.[4] However, if you realize that your estimate was inherently flawed (e.g., you miscounted the number of pages or you used your price list for non-native English speakers instead of a native one), it is appropriate to adjust the fee downward. The fact that you also make mistakes to your detriment does not morally justify keeping a flawed estimate in your favor. (One could argue that the flawed price might still be within the range of market value or cite after-the-fact reasons not to have to change an estimate. However, that approach does not befit the level of integrity you so laudably strive for). Your willingness to forgo questionably deserved money should help you continue to find favor in the eyes of HaShem – and present and future clients.