Or rather, _any_ Leopard pricing can be considered update pricing, since it's not possible to have bought a Mac capable of running Leopard without a Mac OS license.
Apple, in the past, _did_ have a grace period, but since you've already bought the iMac and there was no such free-uprade programme announced at the time, you'll probably have to pay the full (upgrade) price of (quite probably) USD 129.