Yes, it's cheaper to purchase the full CD option pre-cruise. Here's the pricing info that I've gathered:
Individual prices are:
8X10 - $20.95
6X8 - $14.95
The packages are 6 ($99.95), 10 ($149.95), 15 ($199.95), and 20 ($249.95) photos. Different length cruises have different options. You can also purchase an all prints package (all your photos printed) for the same cost as the length of cruise CD.
The length of cruise CD cost varies with length of cruise. The prices I've seen are (for pre-cruise purchase/onboard cost):
7 night $295.95/$100.00 to add prints/$349.95, $100.00 to add prints;
5 night $249.95/$50.00 to add prints;
4 and 3 night $229.95/$50.00 to add prints/$229.95, $75.00 to add prints;
2 night $199.95/$50.00 to add prints
Special CD prices
14 night $339.95/$100.00 to add prints;
12 night $339.95/$100.00 to add prints
The 6, 10, 15, & 20 photo packages can be either prints or the CD with 6, 10, 15, or 20 photos unlocked for your access once home.
Recent reports have the prices for onboard purchases of the above packages at $50.00 more than the pre-purchase price.
I'll also point out, if you purchase a package and it looks like you're not going to get enough pictures for it to be worth it to you, you can cancel the package and just purchase what you want. This has to be done by the last full day of the cruise.