The areas where you can find durian plants are Balik Pulau and Teluk Bahang. There are also different durian name. D15, D11, D17, D600, D700, Hor Loh, Ling Fong Jiau, Ooi Kyau, Xiao Hong, Ang Heh..etc The price per piece is around RM15-25 depends on the grade. Some they sell in kilogram.
For Penangites, you can also book one day tour to the durian farm, have it all day durian there, at the same time you can enjoy nature, learn how to differentiate the durian grade and also how to open the durian. You just pay one sum of money (RM45-50) and eat all you can!
Penang Durian Fair video 1
Eating Penang durian in video 2
About Penang Durian Fair video 2