"Fair price" means third party buyer and third party seller - you have neither. So most likely, your best offer is full price (or a slightly lower amount). If that's worth it to you, then go for it.
That probably sounds wrong, but here is the reasoning:
As the "buyer" what is the highest price you are willing to pay? At what point would you rather just buy a new laptop rather than keep the one you have in your hands now? Most likely, it's higher than "third party" because you've been using it. So don't think of this as a way to get a good deal - there is a convenience premium for you. You are asking because there is a cost for you to switch to a new machine.
From your employer's side, after they replace you would they prefer to have to buy a brand new computer? Or simply hand the next employee yours? Either way the new employee will need to 'customize' it to meet their new job. The advantage to the employer is the warranty terms, maybe less "wear and tear," and technology obsolescence (which on a Mac laptop is pretty low for the timeframe you mentioned) and that's about all. So they have little incentive to offer you a good price on those terms. And if your laptop has sensitive data on it, it might even be a liability to let you keep it.
So, if you're thinking you can "pick it up cheap" then that's possible, but only if your employer is short-sighted and/or under-educated. Since that's entirely possible go ahead and try a low-ball offer. However you should be prepared to offer a near full-price amount for your convenience.
If you were my employee, I'd probably not give you the option to buy - unless you paid full price. If you had a clear "market value price" that was slightly below full price I'd consider it. But the warranty discount is offset by the complexity of the paperwork, corporate data risks, inconvenience of having a new employee spend time shopping for a new computer, etc.
If you came to me and said, "I like my computer, I'll reimburse you the cost for it" that might convince me if I didn't pause to think about the challenges I just mentioned. Maybe there are other factors you didn't mention, but that's the take I get from what you asked.