See article below:

http://www.investopedia.com/ask/answers/06/optionsbuyout.asp

The gist is that you get the value of the option contract, minus the time value.

So, punch your data in here:

http://www.optionsprofitcalculator.com/

Go to the end date of the contract, and the cost for which you believe the company would be sold, and voila, your ROI.