I inquired about the price of medium Durian, it was 2kg @130 baht. So i asked about a small one, it was 1kg for only 30 baht! So i took that one, it was very nice, good texture with a lite coffee taste.  So why the difference in price? I also see 2 large pieces of Durian sold for 180 baht in the fruit market, obviously a rip off.

Not really a rip off. There are different types of durians and some of these are regarded superior to others and can fetch quite crazy prices. Another issue is the ripeness - in some areas overripe durians will be more sought after than the fresh ones. Probably a few other factors in play as well - this is quite a crazy market, especially with the Chinese creating fresh demand.

I read a few months ago that due to the huge demand for durian in China this year, the price for the "king of fruit" has skyrocketed making it too expensive for some Thais. 

 

As shadowofacloud wrote, there are different varieties (300 in Thailand alone https://en.m.wikipedia.org/wiki/Durian) and this probably influences the price/kg as well. 

 

I am rather fond of durian now but I absolutely hated it when I first arrived. 

 

been getting stuck into a friends durian, no chemicals used and the fruit is absolutely delicious, sells at 80 baht a kg I think, luckily we dont pay the asking price. As stated the variety has a lot to do with it and some can get very high prices, really boils down to what you want in taste and texture.