[thejeff]

But it's not 30D for Tarts. (Or even 10 or 15 or whatever it is in the latest CBM.)

It's that for a chance at a usable Tart. It's a really good deal if you've got the Chalice or GoH, but if you don't, most of them are afflicted even if they're not feebleminded. And of those that aren't feebleminded, a good chunk don't have the slots to be SC material or wind up with poor paths for it.

And it's a significant investment for most nations to get mages high enough to cast it. You'll need several casters since you'll need a bunch to get useful ones.

All those other options: You cast them, you get a good SC. Guaranteed. With Wish, you can even pick the one you want. Tartarian Gate is a gamble.
I don't think I'd pay 30D for one shot at a good Tartarian, unless I had the Chalice or GoH.