Membership of the AOI is not mandatory to purchase a portfolio spot on their site, it is a little cheaper however the Standard portfolio is €113 for non-members and the Custom portfolio is €143.80 (For 1 year) http://www.aoiportfolios.com/index.phpoption=com_content&view=article&id=704&Itemid=103
"AOI Portfolios have been developed by The Association of Illustrators (AOI) to promote illustration to the creative industries.
Launched in 2003, we currently feature around 10,000 illustrations by nearly 500 artists covering a broad spectrum of styles, techniques and specialisms. The Portfolios are managed by the individual artists themselves so the content is regularly changing as new images are added to the site."
They offer guidelines for commissions, introductions for newcomers and a spotlight feature.