"point releases" of the current OS, that is, Mountain Lion is 10.8 and the current point release is ".3", so upgrading from 10.8 to 10.8.1->10.8.2->10.8.3 were all free upgrades. However, upgrading to the next version of OS X 10.9 will be a paid upgrade.
Leopard (10.5) was an expensive upgrade, and Snow Leopard (10.6) dropped the price to $29, and Lion (10.7) and Mountain Lion (10.8) dropped further to $19.99 and that seems to be the trend with OS X upgrades, so it might stay at that very affordable price.