NOTE 7/24/25: Darren DeMatas of SelfStartr pointed out that some of the links below are broken. Specifically, WP StoreCart and YAK are no longer in business. You can see a recent comprehensive list of WordPress e-commerce solutions on Darren’s website.
Premium (gold cart) version is $40 for a single user, $195 for developers. There are a number of other commercial add-ons to add more functions.
Commercial plugin: single license $55; developer license $299.
Commercial plugin: single license $89; 5-domain license $179; developer license $399, all per year.
Pro version is $29.99 for a single license; $119.99 for a 10-domain license; $209.99 for a developer license.
(No good screenshot—there is no styling whatever on the plugin home page.)
Be sure to read the FAQ on the WordPress plugin repository for the necessary shortcodes to insert products into posts.
Plugin is free; documentation is $25.
Commercial themes: $65/year standard license, $99/year developer license.