Dan makes a good point (as usual). It's not just the lower values, but the fact that you're narrowing your market, perhaps significantly. A lot of people just won't live in such properties regardless of price. Others, though, won't mind. In areas near me (and I concede that all real estate is local, so the numbers might well be different where you are), there's probably a 10%-15% price difference . . . which really is pretty much in line with what Mary Beth suggests.
Still, if the power lines don't bother you and you're getting a good deal (based on comps for similar homes--especially those backing up to power lines) and you plan on living there for maybe 6 or more years, go for it. If other homes in the area appreciate, yours will too. And while your price might be 15% lower when you sell, you'll have bought for 15%+ so that should be a wash. Just remember that not as many folks will be interested in such a property, so it could take longer to sell.
Hope that helps.