the $90 is just the unit without the hard drive. You'll want the ones with the hard drive already with it, so about $150.
Just for comparison, I took about 3000 pictures on my recent 9 day trip, each picture was a 3mp superfine setting - the highest resolution of my camera - and each picture was about 1.2 mb and for approx 3000 pictures I used up 3.7gb hard drive space.
So a 20gb storage unit can literally save 10-15,000 pictures depending on the size of each picture.
I think it's a good investment for $150 - if you burned your pics to CD at Disney at $12 a piece and a total of 120 pictures per CD like I've heard you're looking at 10-15 CDs for the same amount. With a portable storage unit, you could use it for digital pictures straight from the memory card or when hooked up to your computer, you can use it like an external hard drive. IMO a much better deal.
The unit comes with slots like a memory card reader, and you literally just pop in your full card, press a button to copy it to the unit, and then you're done. It'll tell you when it's done and then you can just put the card back into your camera and delete all the pics from the card. These units take the standard cards, like CF, SD, Memory sticks, etc. Usually it'll take xD cards with an adapter so read the description carefully.