The new weapons seem to gain more stats when you upgrade them, or they have their own unique things like Procyons' Stagger Maintenance. But it does seem a bit of an odd situation.
Edit: Rye found a thread online somewhere which said you need 36 of the cheap items - 80 gil ones from Creature Comfort - to get up to triple EXP. I've not yet tested the extent to which this is accurate but so far it's serving me well.
We found a place to grindRight now we're in Chapter 9 and we're fighting a load of enemies who drop Credit Chips and Incentive Chips, which are very good money. There are a couple of save points with a screen change in between; it's a great place to grind out some equipment levelling.