I'm starting to interview for private practice jobs right now and come from a "Top tier" institution, whatever that means. Broad, general numbers, full-time position in a good group (not a national corporation i.e. 21st century or USOnc):
Starting salary: $200-$300k (not including signing bonuses, which can be around $50k or so)
2-3 years out, making partner and obtaining full "professional fees": $350-$500k, depending on volume and billing. (Professional fees are the fees that you collect as the physician providing the care to the patients.)
If your group owns the machines, real estate, etc, and lets you buy into this component, the "technical" compnent: $750k-$1.2M+. (This is where the money gets, frankly, a little embarassing.) Keep in mind, though, that the size of a buy-in (money you give the group) for the technical component can vary widely, from $250k to $2M (!).
If you do sign with a larger, national group, your starting salary will likely be higher (i.e. $350-$450k), but your ability to see the huge #s is zero.
Never, ever let anyone tell you that you have no right to ask or know these numbers, and never feel "bad" if reimbursement is part of the reason you wanted to go into radiation oncology. It is a very long, painful haul to become a physician of any kind, and wanting to know what the financial "light at the end of the tunnel" will be is perfectly normal.