All of the chinese restaurants I use increase the price of a dish very slightly (by about 20-50p) if it's a king prawn dish rather than a chicken, beef etc one. My local Indian restaurants increase their price a lot more (between £2 and £4 extra depending on the dish), is there a reason why there is such a big difference between the chinese incease and the indian one other than "because"?

Depends whether they are using fresh King Prawns or pre-frozen imported versions

Yes that is a possibility.

they probably both use frozen ones. id say its because in general an indian is usually more expensive any way so they know they can mark up a little bit more.