Of course, no one should be asking for prices, since Wordpress is GPL software and thus all plugins should be released (that is, if they are released) under a
GPL-compatible license (says so in the
WordPress codex.) This means that if you're distributing your plugin, you should release the source code to the public.
Hate to be a party pooper, but if we're using open source software, we should play by the open source rules. I've written a few custom WP plugins myself so I had to read a bit about the licensing rules, and this is exactly why I've decided not to distribute them to anyone else (yet)...