I also have a mossberg 500 in 12 gauge (no scope though). In the middle priced stuff i've had pretty good results with both hornady sst's and I also like remington copper solids pretty well. If you watch for a sale you can usually get them for $10-12 a box, at least the last time I was buying you could.
I tried the copper solids in 3" and didn't really notice much difference in recoil
They don't tend to go on sale as much though.
Lightfield slugs shot ok but I couldn't see paying premium prices for plain old lead slugs. They cost as much or more than the hornady sst's and remington copper solids.
Personally if I want cheap practice I just shoot regular rifled slugs out of it, like federal classics or remington sluggers. At 50 yards the point of impact is about the same, they just group a little bigger. You can shoot a couple boxes of them to get some trigger time on the gun and then finish up with whatever ammo you plan to hunt with to make sure your zero is good and everything.