With the fronteir play discussion going on, I thought I would throw another "dry box" into the cheap category.
I got a couple of these for $19 each on sale a few years ago and have been using them for work dryboxes and for river trip "mostly dry" boxes. They have an lip and foam gasket. The boxes are stiffer than action packers and you can sit on them with much less flex than the action packers. I have had them on several week long trips, through class III and IV rapids with no leaking, but I am sure they would leak upside down.
I have been using these mostly to store my kitchen gear, stove, and propane grill on river trips.
A good cheap option to get into a pretty water proof box.
I did have to slightly modify the boxes by cutting off a little bit of the "fins" on the outside so the box would sit lower in the frame bay.
Most noteably they are dry in the upright position taking pretty good waves over top and I have abused them fairly significanlty and they haven't cracked.
Outdoor Storage Box - Snap-down Locking Container - Box Storage - Garage Container - SIA-760D