It is not expensive, but I strongly recommend against trying to do it yourself. It is not practical to properly charge a R134a system by gauge (see how to do it if you have a dynamometer climatic wind tunnel, first source) and overcharging carries significant risk of damaging or destroying your compressor. In my opinion, all the mess-it-up-yourself kits should be removed from the shelves.
It costs about $100 to have the A/C serviced properly by a professional, and anything else is folly.
www.autoacforum.com