Nope - same price - we bought those before the cruise last time too - hubby decided he did not want to go so I used both days.
Truthfully the reason that I buy things ahead of time or put on board credits on is to get the 6 months no interest on my Disney Visa - see if you put it on the card BEFORE you go on the trip you get this benefit - but anything you put on the card ON the trip doesn't - I found this out after our first cruise - and by doing it a month ahead of time for the cruise we also get disney points to spend on the cruise from our things we prepaid.
Now here is what I prepay
Tips
Water Package - it is nice to already have it in the room when we get on board (we do not really drink alcohol but we do drink water and I know the water on board is just as good but this is something hubby wants to do so we do it - I will pay $1.66 per bottle for it - but refuse to pay $65 for a bottle of wine I can buy for $20 stateside (we both love Rosa Regale and will bring a bottle of it for the cruise)
after that I just add up the money for the following and put it as an OBC:
Pictures
Excursions
Spending money for the kids
Spa treatments we have booked
Palo meals
and a little bit for extra's
This helps us budget also - then on board we use a different credit card that gets us points for other things if we go over
Krisann