How much would it cost to replace a heating-oil system with natural gas?

I was wondering how much it would cost to rip out an existing heating-oil based system and oil tank, run gas lines to the house and put in a system that used natural gas. I have a friend who could probably help do some of the HVAC labor for cheap, so that would help.

Obviously there are a lot of unknown variables involved here, but I was just wondering what a ballpark idea would be, between 5k and 10k perhaps? I’m house hunting, and in my price range there are a lot of 50’s era heating-oil with forced air based homes, so it’s a bit of a concern because I’ve heard they can be a real pain to deal with.

Anyone have any experience with a conversion like this? Are there any hidden “gotchas” I should be on the lookout for?