Supply and demand...they produce many more rounds of 9mm than 380, and recently there has been a very large move towards 380 as carry guns so ammo is more in demand.....thus prices are higher.
My wife and I both picked up 380's recently and now I need dies & bullets to start reloading this caliber.
Don't know what you're paying or how much 380 you want to buy/shoot but these folks have some decent prices. I have gotten orders from them the next day. I know this doesn't answer your question but Magyars pretty much nailed it. I buy most of my 9mm at Wally . Our Catalog
The ammo pricing never seems to line up. For example, if the same bullet is used, it only costs 1 cent per round more to load .357 than .38 spl. When was the last time you saw .357 only cost 50 cents per box more? It is also bad with shotgun shells. 12 guage shells with 1-1/8 oz lead can be bought for $5.00, .410 that only have 1/2 oz lead cost $13.00. If you shoot 28 gauge, that problem is way bigger than that. That is why I reload.