You need to speak with a good accountant and/or attorney. Irrespective of the cost of the buy-out (even if it's $1), there are definite liability issues you assume when taking over a corporation. That's the stuff you need to be particularly careful about.
As far as the value of a strictly professional corporation, it is really tough to estimate that for Rad Onc. I definitely would not agree with the usual multiple of gross calculation people use in practice buyouts in other specialties. The big issue with Rad Onc is that you are so reliant on someone else to provide a location for you to practice. If you lost the PSA with the hospital or the damn hospital burns down, the corporation and contracts are worthless since you'd have nowhere to treat. That being said, taking over the corp may be the only legitimate way to break into an area, so I think there is some inherent value to it. There are some locales where many of the rad onc insurance networks are closed to new physicians. They will, however, usually allow you to add a new physician to an existing contract.
Perhaps you can tie the buyout to the availability of a center to treat. It may be a long shot, but you could offer "x" over a certain number of years, but the repayment would be nullified if you lost the PSA or didn't have a place to treat patients. I've seen one buyout structured like that.