How many machines can I install the $79.99 version on?

If this is a personal purchase, not for a business, then the terms of sale for the Mac App Store state that you may install any software on all computers you "own or control". If it's for a business for installation on multiple computers that are each used by a different user, then basically the terms are one installation per purchase. Here is the official statement:

If you are a commercial enterprise or educational institution, you may download a Mac App Store Product for use by either (a) a single individual on each of the Mac Computer(s) used by that individual that you own or control or (b) multiple individuals on a single shared Mac Computer that you own or control. For example, a single employee may use a Mac App Store Product on both the employee’s desktop Mac Computer and laptop Mac Computer, or multiple students may serially use a Mac App Store Product on a single Mac Computer located at a resource center or library. For the sake of clarity, each Mac Computer used serially by multiple users requires a separate license.

Regards.