Real Estate Gurus: just how bad are power lines for resale?

I’ve found what looks to be a really great home at a fantastic price, but the catch is that there are power lines that run along the back yard. I realize that they don’t have any negative impacts on health, but I also realize that many people hold the erroneous belief they do.

So just how big of an impact do they have on your property value or the ability to sell? Do they tend to sit on the market for very long periods of time? Is it equavalent to having a hazardous waste facilty nearby or an airport? Can a rough $ figure be attributed to them (e.g. powerlines means -$50k off the price)?

Thanks in advance for your help!