Estimates are the cost to Verizon ranges between $650 to $9000 per house to wire up FIOS (yes, I know that is a big difference, but you can see here for more details):
Will Verizon's Fios plan kill off the company? | Computerworld Blogs
Needless to say, the more houses on a street and the more fiber in place the more likely the rollout will be.
I think also because FIOS can deliver cable tv, there are issues with local ordinances that grant tv monopolies to certain companies. FIOS could threaten those laws / contracts so in some places Verizon may have to wait for some changes in the law