Even a nicely finished basement isn't worth nearly as much as above-grade space. Susan's probably right on her figure, but I'm guessing that the figure applies to nicely finished (i.e., a ceiling, paneling or drywall on the walls) basement.
I'd be more inclined to value it, at most, at the material/labor cost that went into it. Maybe $5,000-$7,000.
But your agent would be the best one to advise you on precisely what value (if any) that basement adds.
Hope that helps.
What you can't put a price on is the value of your enjoyment as a buyer. We personally built a home with a basement knowing the basement won't be worth as much as the rest of the house, however there is tremendous value to us on other levels...such as a space where we can have a media room, playroom for the kids, bedroom for guests, full bath, etc.
You said you are looking at a home with a basement...what does your real estate professional think?