From now until April 25th, 2013 you can get them for $9.99/€8,99 each on the App Store. That's 50% off the normal price of $19.99/€17.99.
You can also get the Piano Expansion Pack and the Electric Piano Expansion Pack inside each app for $4.99/€4,49 through in-app purchase. Already have iGrand Piano Free for iPhone or iPad? Well you can upgrade to the Full version at the special $9.99/€8,99 price through in-app purchase as well.