Rather new to arbitrage, these are the neutral pricing points for this position. But you get a pretty clear view of why the stock is at this position by looking at the probability of the success of the deal and the worst case of the price dropping back to $10 if the deal falls through.
10% Chance of buyout deal failure Stock Valuation : 20.8
5% Chance of buyout deal failure Stock Valuation : 21.4
2% Chance of buyout deal failure Stock Valuation : 21.8
Couple this with an estimated 4 month period to recoup any gains for the buyout yields an approximate rate of return of 1-15% for this position (1%-10% deal failure, 10%-5% deal failure, 15%-2% deal failure).
The stock price therefore appears to be fairly valued considering the conservative risk of going back to $10.