It depends on the location. If you are talking about downtown Manhattan you can get dark fiber between any major buildings for about $5k/mo. If you are not in a mojor building then Zayo would do it for about $20k build out worst case scenario + same $5k/mo, since Zayo touches a boatload of manholes. There was also Metro Optical but I dont know how large is their footprint. They are quite creative.

But in general, go to a decent building and order Cogent. That gets your building on the net. Cogent eats fiber extension charges left and right. Plus you actually get 1Gbit/sec for $500/mo. Now you take that 1Gbit/sec and wire to neighboring buildings by running ethernet or standard yellow fiber. That's how ghetto ISPs get stuff done. It was done for years because it works. There's incentive for those who own building to form a community ISP or provide an access to a community ISP because those that own builds want the same product and they know they can share some of the costs.