This pretty much sums up my limited experience. There are always one or two people who disregard the price limits in the upward direction, leading to a few coveted items amongst the dross (and probably insecurity on everyone else's part who did follow the rules) -- and then one or two people who disregard the price limits by severely undershooting (the year my parents went to one such party and returned with wind-up "chattering teeth" remains the ur-example in my consciousness), creating discontent on the part of everyone else and throwing wrenches into the fun of swapping and stealing.

Not a direct experience of mine, but I've several times heard of people whose group did "Ornament Swaps", where all items were Christmas ornaments; that might be an attempt to even the playing field somewhat, along the lines of commenters' suggestions that items must be 'useless' (because let's face it, who doesn't have A. enough ornaments already, and B. no functional use for them eleven months of the year?)