I suppose the question really is, could that extra $60 be used somewhere else that would benefit the SEO more then hosting the sites on different IP's.
Personally I would go with one server and use the $60 on SEO, ive seen a video from Matt Cutts saying its fine to inter-link websites but make sure they are relevant to each other.
If its a directory you must have been collecting email address? Can you not use the email db to expose the new directory to these users?