You would initiate a transfer at the new registrar you wish to switch to, so the price is essentially the 1-year domain renewal cost at the new place. One year is added to the expiration date of the domain once the transfer is successful.
There are many registrar you can go with...GoDaddy.com is one of the least expensive (around $8/yr). Personally, I have a reseller account with Tucows/OpenSRS ($10/yr). eNom and Dotster are also widely used. Fabulous.com is nifty too. See:
http://www.internic.net/regist.html
for a list of all accredited registrars. Most would be $15/yr or less, these days.