I've not noticed 10mm being substantially less expensive than .357Sig- usually the reverse.
I'm guessing it's a local case of supply and demand. 10mm Auto is an uncommon niche caliber. Relatively few guns are currently available in 10mm and I'd surmise that few have been sold overall compared to guns in .357Sig. For whatever reason, I'm guessing your local gun stores wound up with too much 10mm ammo and can't get rid of it all, hence the lower price.
FWIW one possible factor is that some police departments adopted the 10mm as their service cartridge soon after the FBI did in 1988, then dropped it after they realized that many of their officers couldn't qualify with it (like the FBI). Some of these departments were left with big stockpiles of 10mm training ammo that have been sold as surplus to local gun stores, creating local gluts. If the cheap 10mm ammo appears to be 10-20 years old, this may be the case.