Yeah, now that I think about it, I think it was the ones in the Magnetite vein, which the wiki says is worth four times as much. Could be that there was some platinum in there I forgot about, too. (Come to that, is there any way to make them cheaper, or should I just dig it all out and smelt it?)
The value of magnetite should be 10, which is actually 5 times as high as limestone and other flux, and 10 times as high as "generic" stone. What this means is that rooms dug out from magnetite, not to mention other, even more valuable ores (you mention platinum, which has a value of 40, if I'm not mistaken) will be pretty damn expensive.
Digging it all out will still leave you with floors of magnetite, and I think those are exactly as valuable as walls. Covering those magnetite floors with floors made out of less valuable stone is the only way I can think of that might possibly make those rooms less expensive, but I don't think it would work. It's more likely to just add the value of the newly-made floors to the magnetite floors beneath them.
I think you have only two realistic options:
1. Dig out rooms in limestone or whatever else you can find that's less expensive than magnetite, or
2. Disable rents. As someone who never plays with economy on, I don't know whether that will even work post-facto.
Where can I get me some ore? I mean, all I seem to have is gems, two pockets of platinum (on the bottom level at least, I'm strip-mining the whole level), and a decent bunch of tetrahedrite but no copper or iron ores... I think I marked flux on the finder, so that's probably there, but my stuff sucks.
Just to clarify, tetrahedrite is a copper ore. You should look on the wiki or in your raws for info on which stone layers contain which kind of ores. Or you could do it like me, if you're greedy, and mod your raws to contain more ores. PM me for details, if you're interested (I'm not gonna post my little "guide" here, as I've already posted it a couple of times on these forums, and I don't want to bore people).