You don't mention how much cash the company raised, or how much cash is left. These are key questions if you want to know how much of a buyout is appropriate.

If there's not a ton of cash, they are going to be very hesitant to give much of it to a departing founder.

At the same time, you should be hesitant to just hang onto your equity on the promise that it will be bought out later. If the company isn't successful, you'll get nothing. And if the company is successful, future investors will convince your founder that you don't deserve anything and that they should do shady stuff that will dilute your shares to nothing.