Cons: I like putting the bad right up front cuz I'm a mean old man.
Cost is number 1. Mine cost $17 off e-bay with free shipping. A plastic box with a 1/2 thick piece of foam sponge and a 50 sheet pad of palette paper cut to size. Box dimensions are about 6" x 5.25" x 1.75". Refills for the palette paper around $11 for 2 pads of 50 sheets.
Paper drys out:
Stuff kept drying or curling. I tried adding more water and then the paints ran together. Also, when the paper gets wet it increases in size and no longer fits well within the box, allowing air to come in more readily and speeding drying. I did find a couple of work-around for the paper problems, which I haven't tried yet. One involved using magnets, the other was to place a moist paper towel on top of the foam and then place the paper onto that. I should mention, though, that even though the paper kept drying and curling, the paint was staying moist. So maybe more of a nuisance than a real problem.
Pros: Grudging acknowledgement that this is a pretty good gadget.
Size - A nice size for me because my painting space is so cluttered up so a small footprint is okay.
Keeps paint wet. It works well. I did some painting yesterday morning and just closed the box. When the lid is closed, the paper doesn't dry out, nor does the paint. Here's a pic of yesterday morning's paint this afternoon.
Buy one of the P3 Wet Palettes if you've got money burning a hole in your pocket, they're pretty good.
Or make one yourself.
Dollar Store reusable plastic food storage container of a size you like. I got a pack of 4 for $1.
Dollar Store foam sponges: 1 package with 2 sponges for a dollar.
Paper towels or napkins: I had some already but readily available for a buck.
Reynolds Wrap Parchment Paper: 15 inches wide by 24 feet long, Walmart $3.