It would be quite expensive. You should just do it yourself.. buy laptop RAM from any PC store and put it in using your own tools + means. It's a lot cheaper and also much more fun to do it yourself I did it to my Macbook and it took about 10 minutes.
Installing yourself is pretty easy, you can find many guides online, just be sure to find one for your model. Apple would likely charge you at least $200, maybe more, and you can find 4GB of 1067MHz DDR3 for less than $100.