It's more of a personal design philosophy thing, I guess. I know that, by the book, you don't reduce the additional costs at all, but that leads to items that cost far more than they are truly worth to the players.
Look at the ring I mentioned to see why I do this.
It gives a +2 enhancement bonus to all six stats. Okay, by the book, this is 4k for the first stat, and 8k for each other one, for a total of 44k. This is the same cost as if I bought a +2 slotted item and five +2 unslotted items for the other stats.
Now look at it from a practical point of view. This is actually not as good as having the slotted/unslotted combination. If the other five +2s were truly unslotted, I could replace each one once I find a +4 item. If I'm a Wizard, I'm not going to settle for a piddly +2 INT item when I can afford better (it IS a 40kgp item, after all, more expensive than a +6 slotted item). So, while I'll buy the +6 item eventually, I've now wasted 8k on a +2 INT boost from the ring that doesn't stack. And, to make it even worse, I would have rather upgraded the INT item myself, instead of buying an entirely new item.
Likewise, as a Wizard, I really don't care about the CHA boost. What I'd REALLY want would be an item that boosted INT, CON, and DEX. Instead, I'm being forced to pay for abilities that are incrementally less useful. If they were six separate items (5 of which were unslotted), I'd simply choose not to buy the STR, WIS, and CHA unslotted ones in the first place. Each of those would save me 8k, and with 24k I can buy a lot of Quaal's tree tokens. But, I don't have a choice, I HAD to buy all six stats, in a form that can't be effectively upgraded.
So, I called that 35k instead of 44, add another 5 for the Ring of Sustenance effect, and called the final item 40k.