If really 50 photos per 15 mins like that, it can be only be very basic retouching or batch processing. The time spent is like for example a few minutes on the retouching setting which should be same for every photo, and then apply to all which would take the rest of the time to process finish.
If that is the case, I think the client should be able to handle it all alone already. So, from this I guess your client wants you to charge cheap, based on his example. However in order to ensure the quality of each photo, you will have to go through one by one, so your rate can be reasonable low but the time you spend on retouching should not be as short as 15mins for 50 photos.
I guess your client should find a slave for such demand.