as for desktops just to go Khaled Bin Al waleed street and check the prices !!! usually u select the specifications and parts u want to assemble for a desktop u should think about specs rather than price, as for laptops, it is simple also !! near Khaled Bin al Waleed street go to Al Ain centre !! there they have the best prices for laptops. Personaly I purchase from Care Computers on the second floor !! there is a nice guy there called Hardeeb ask for him and he will give u a good price !!!
Good luck !