Many of the custom projects I take include things that could be useful to the open source community. I'd like them to open Github repos with them :)

Most of them are jQuery plugins. I'm almost always ending up creating my own jQuery plugins for features I need to implement in the project, for example autocomplete, edit-in-place functionality etc. 

Would it be a good idea to offer a "discount" for people that accept to let me open-source part of the project? Like -15%, -25% etc?

Do you think it would be unprofessional to ask them this?