If your product is covered by SmartNet, then I think that the upgrades are free.
Here's the extract from the SmartNet Q & A
Q. Are software updates and upgrades included in the Cisco SMARTnet charge?
A. Yes, software updates and upgrades within the customer's current platform are part of the basic program. No additional charge is warranted as long as the product remains under Cisco SMARTnet.
For more Information, refer:
http://www.cisco.com/en/US/products/svcs/ps3034/ps2827/ps2978/service_q_and_a09186a00800b7706.