It really depends on the metro area. Here in Chicago I can get a dark pair about 3 houses down the block from me (never quoted out how much it'd be to extend to my address) going back to 350 Cermak. This ran about $600-1200/mo or so depending on contract terms and who you know. If you have other gear in the facility (e.g. transit and routers with a spare port) you're looking at another $150-350/mo (again, depending on your contract) x-connect to get into your panel in your cage.

Given I have equipment already in the datacenter, it's pretty tempting to pull the trigger on a 100G link. I just have no idea what I'd do with it.