Behind the gas tank is a fairly common place for them to rust out. It's not that bad to replace, you need enough brake line to reach from the rear (where it connects to the rubber, just behind the tank) to the front where it connects to the rest of the system. After replacing it you just have to bleed the brakes. You don't have to drop the tank, you can just slide the new line in between the tank and the frame. Probably looking at $20 - $30 in parts, depends if you get bulk line and flare it yourself, or get pre-made lines and join them together with unions.
If you take it in to a shop, I would estimate about 1.5 to 2 hours labor plus parts (~$20-30). The total price depends a lot on labor rates in your area. It's really not that big of a deal.