250gb? How many image files do you have that size? Bloody hell!
Oops. Yeah, I meant megabytes. Error fixed!
In late 1990 I used PS v2.0 for basic background removal using the Lasso tool. What a PITA. I used a IIci with a removable Syquest disk. I forget its capacity. I received CMYK TIFF files from color separators, and while most files were 20–40MB in size, sometimes the project was a large box top for the product with a typical image size of 36"x14" @ 300 ppi. Some images took up to an hour to save to disk. And with only a single undo, I didn't save too often with those large files.
I was very
grateful for PS v2.5 because it shipped with the mighty Pen Tool.
The point I'm trying to make, though, is that charging an hourly rate is not profitable when you're a fast, efficient worker. In addition, at some point the skill level predicates a level of mastery that would make an hourly fee appear inflated.
If you charge a flat fee per image, you are free to try things and experiment, which greatly improves the chances of producing better work. And at no additional expense to the client.