Where the price goes up exponentially is when:
1) You hit rock or water while digging, which requires special equipment and huge additional cost
2) You have to build a road to access the site
3) You have virtually no access to the job site at all (remember that some LA signs were built using helicopters)
4) Cities that are unionized and ridiculous in the red tape and regulations (LA, Chicago, NYC)
The most expensive signs are those that are full-flag offset, and price also goes up as you get higher, as well as the size of the V.