It doesn't cost rubies, it costs money. Scholar can currently be obtained only as a part of some "ruby specials". You buy rubies (for money) and you also get ships and/or various items. Among them, Scholar may be included (if it is available, it will be clearly emphasized).

Also, note that you cannot get another Scholar if you still have one (active or inactive, doesn't matter).

Last, but not least: if you do get a Scholar, shortly before it expires click the two longest lasting upgrades. They will continue in parallel after the Scholar expires, thus giving you the most out of it.

Thanks!