The risk of unforseen broken tiles sounds reasonable but that attempted repair in secound pic sure isn't.

What's the average cost per slate tile?

An average cost per tile would vary tremendously from place to place. It is also a rather odd way of pricing a slate roof repair. Slate roof maintenance and repair require a high level of specialized skills, so are usually priced by the job, not by the individual slate. Damaging 20 to 25 slates in order to replace seven seems pretty excessive. Damage to existing slates most commonly happens when the person doing the repair walks directly on the roof. Good slaters don't do this. They use a ladder hooked over the roof ridge or perhaps one of the hydraulic lifts commonly available today. Accidental damage to a few slates is possible even with the utmost care, but not 20 or 25. Your best course would be to get an estimate from a company or individual that specializes in slate roofs. Lots of roofers claim they can repair all kinds of roofs but frequently lack the knowledge and skills to deal with slate. There are very specific requirements for flashing and nailing needed to ensure a good permanent repair.

It is nearly impossible to predict how many slate tiles will fail or need to be replaced.