Hi,
This is a great whitepaper
Cisco Catalyst 3K-X Service Module: Enabling Flexible NetFlow in the Access
Pls note it states
The minimal Cisco IOS Software release for the service module is 15.0(1)SE. Older Cisco IOS Software revisions will not recognize the service module.
Here you can find service module Primary Features and Benefits
http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/product_bulletin_c25-688868.pdf
Regarding the price, it depends on where you buy it from, but it should be between 2 and 3 K$. You'd better ask your Cisco reseller.