The Sharn sourcebook contains a Lv17 commoner. Basically, she's an old elf who's dedicated her entire life to becoming a master cook.
IMO, high-level commoners wouldn't be especially common, but they would be especially good at what they do (unlike an expert though, they would have an extremely narrow field of abilities given their 2+int skill points/lv). They would be more common among the older-lived races and would all probably be mostly old, and probably have natural skills in their profession. Even more so than others, they probably particularly value hard work, and few days would pass by when they didn't work in perfecting their craft. And of course, they'd probably have to love engaging in their craft. Even still, these are exceptional individuals (just not exceptional adventurerers).
How much gold they get is really up to you. Do you want them to be mostly unknown and out in the middle of nowhere? Then you can get away with not giving them much. Do you want them to be owners of a globally known business? Then it'd make more sense for them to have the starting gold. As for CR, that'd depend partially on how much treasure they have (and, as noted, the standard rule for NPC CRs doesn't work well for commoners), so you'd probably just have to wing it. Don't worry too much about how the EXP system works -- just assume that they acquire levels differently from PCs and they're so good simply because of their extremely hard work and excellent innate abilities.
What would motivate a commoner who reached that level of awesomeness? Reaching an even higher level of awesomeness. There's always more problems to deal with, or new breads to make. If you want to, you can even have them aim for epic level, though certainly give them appropriate feats (Epic Skill Focus is always good). But as for why they'd join together? Well... that's really up to you. I personally wouldn't make commoners of such a high level common (no pun intended) in my game, so I can't see them meeting often, if ever. Perhaps a particularly rich aristocrat is seeking out servants who are the best of the best? Or there's a very special race/society that generates highly skilled people?