Depending on how the market looks like, prices are set. There are still a small percentage of people who buy snowballs for that ridiculously high amount of Gigas. Sellers are able to so 'cause sooner or later someone will purchase it.
It's a matter of time; how fast the seller wants to get rid off the snowball, if it's as early as possible the price will be lower obviously. Contrary if the player has plenty of time to sell the item (and/or are not in a desperately need of Gigas).
It's the same principle for a buyer as said above. Also if the player feel like a snowball could be very useful, a high price will only be a small factor. For the purpose of changing name as Grim said, or for some grinding perhaps, supposedly when it will pay off the most - while EXP rate is as high as it can be.