2) It has the same drop rate as the others (according to wiki:http://wiki.guildwars.com/wiki/Miniature_Pig)
3) Its initial numbers were reduced due to the celestial pig exchange in 2008.
Adding all this up, there are presumaby less mini pigs than celestial rats or oxes in game. Yet they are priced around 15-30k while rats are well over 100k and oxes over 60-70k...
It is kinda odd if you think about it

Is it its looks?