Each premium theme has a different price so you buy one and if your don't like it then you request a refund and you purchase another premium theme.
WordPress.com provides a 30-day refund on all upgrades except Domain Registrations, Domain Renewals, and Guided Transfers.
Provided you are logged in under the same username account that purchased the upgrades, you can request a refund yourself directly from the Store section of your Dashboard
http://en.support.wordpress.com/upgrades/#refunds
Note: It takes from 1 - 2 weeks for the refund to be received.