hm, OK. in that case, I'll give Apple the benefit of the doubt and say that if 10.1 and 10.7 could possibly run on the same computer (since 10.1 was PPC-only, and 10.6 and 10.7 are x86-only), they'd let you upgrade directly for $30. thanks for the clarification. :)
(though, I'd still argue, an extra $70 per decade is not exactly breaking the bank.)
I don't know about the overall upgrade pricing for OSX, but if you wanted to go from 10.4 to 10.6, they required you to buy the full suite with iLife and all for $129 or whatever it was. $30 upgrades are only supported if you upgrade every time.
(though, I'd still argue, an extra $70 per decade is not exactly breaking the bank.)