It depends how much of your stock was in return for the money you put up, and how much in return for your work as a founder. If your work as a founder was worth zero, the valuation of the co (at the moment you invested) would be $100k, since you bought half of it for 50.

In most startups you don't have to buy people out if they leave (in the first year) because most startups have vesting with a one-year cliff. I.e. none of the founders get any of their earned stock till they've worked for a year.