Does it cost much to run the blackberry enterprise server?

1st what kind of BB devices in your environment2nd as Muhammad says it's Depending on the number of user they r using BB.

no

Depending on the number of licenses required.
The cost of deploying BES5 is very small.
Dont know much about the new version,10 though.

If you have high corporate account with your mobile service provider they can provides you with BES express with limited number of license about100 after that you have to upgrade your quota , but it’s may vary per regions or per service providers offers.