Even if it only takes you 30 minutes, you need to consider the headache and increased risk associated with a client that files an amended return each year. For a client like that, there should also be a "punitive" element, for lack of a better word, internalized in the fee to incentivize them not to operate like that. IMO of course.
If they just gave me garbage the first time around and the return needs to be completely re-prepared, I could see the amended return fee surpassing the original return fee.
If you want to keep them as a client and not fire them that is.