Here's a link to Remodeling Magazine's Cost vs. Value report for renovations and additions. The data might be helpful. http://www.remodeling.hw.net/2009/costvsvalue/national.aspx
I find that 3-bedroom homes are now much more in demand than 2- bedroom homes. Even single people seem to want a master, a guest room and den/office. As you've described it, the current basement would likely detract from the value of the house. For both of these reasons, I'd say the value of your house will increase. A local agent can help you with how much the incease will be.
What year was your house built, where is it located, what condition is your home in? I need to ask you some more questions to give you a realistic number. 612-423-0202
Adding a basement bedroom and bathroom does not automatically add value to your home. It depends on the market in your area and many other factors.
Contact a REALTORÂ® for more specifics on the value of your home. It is not something that can be determined without much more information.