Is there a good way to host tiles for only a small part of the world? I have some apps that are only useful in my county - so I would like to have very detailed tiles for areas in my county, but there's no reason for me to generate or serve street-level tiles for somewhere in California or Tokyo or wherever.

Because it's hard to lock down tile servers for public-facing apps, I have always been a bit nervous putting anything online in case someone swipes my key and uses it and one morning I wake up to a huge bill.