Been doing this for years with my previous MacBook Air's and Pro's I've owned in the past:
http://apple.stackexchange.com/ques...rd-grease-from-a-macbook-airs-normal-keyboard
Basically, 3 parts rubbing alcohol and 7 parts water. Diluting the alcohol in water helps to decrease the "harshness" of it with regards to contact on your skin after cleaning. In terms of the keyboard itself, it won't damage anything, and helps to dissolve the grease. You can read the link above for more info.
I believe a lot of commercial keyboard cleaners use the same or similar solution above, so you can DIY your own solution for a fraction of the cost.