At the places I interviewed, to my knowledge, tail insurance was graded depending on how long you were with the group.
If you do a search you should be able to find the calculation for the cost. I don't know it off the top of my head. It's some fraction of the premium. Either way, it'll be expensive.
Depends on locality, state laws, how long you've been in practice (tail after 6 months cheaper than after 5 years) and how long of a tail you want. Something in the range of a nice new car.
The answer is that it depends. This is something that you need to discuss with the practice - and the practice may in turn discuss it with the practice manager or consultant.
Roughly, the first year premium may be $4000 (depends on many circumstances), and then rises $4000 a year for two years to $12000 (where it stays). When you leave, the cost for the tail is around 2x the annula premium you are paying.
Also, the $4000 figure I used could be substantially different based on the insurer that you are dealing with. If the agent does a significant volume of business with a particular carrier you may be able to get a significant discount (40% or so).