Sometimes I undercut by several thousand, just for lols.

Depends on the market at the time, though. Sometimes I undercut because I feel price gouging is going on. 6000 gil for a level 30 item? Thats a bit steep. Try 3500. Sometimes if I see people being petty and undercutting by 1, I will halve the price and mass produce it just to drive them out of the market.

It all depends on how bored I am.