I purchased both 27" iMac and MacBook Pro on March 2012 wth Lion OS installed. I just upgraded to the OS X Mountain Lion for my iMac and was wondering whether I have to pay again for the upgrade for my MBP?
no you do not need to purchase it again. You can install it on multiple macs you own. Sign in to the app store with the apple id used to purchase ML on the mac(s) you wish to install it on. Hold down the option key and click on the purchases tab and click "download" next to ML.