$600 for a sticky spacebar? I spilt a little bit of juice on my Macbook Pro keyboard which causes the spacebar to stick (it still works but is not so convenient to use). I took it to a non-official Mac service where they told me that it needed some cleaning that would cost around $60. Then I took it to an official Apple service where the guy without any deeper investigation told me that a brand new replacement top-case is required for $600.
The computer is otherwise completely functional. I still have a warranty until February that would probably be void if a non-official service fixed it.

Do you think the $600 offer is realistic and this much has to be paid just to fix a sticky spacebar?

Update: my girlfriend accidently spilt a half glass of water on my keyboard, and after drying it, the stickiness vanished :D Gotta admit, Macbooks are of high quality and I should care more about my computer.
That $600 rate is less than Apple charges for a liquid damage overhaul at the repair depots. That repair is around $900 for the Air and 13 inch models and $1200 for 15 inch models.

Since the keyboard is integrated within the housing, it's like taking the engine, chassis and all parts out of a car and building it up new in a new frame.

The labor time and consumables (ribbon cables that are too fragile to reuse, etc...) seems about right for a $600 repair. Apple techs see this sort of things all the time, and won't push the expensive repair unless there is internal damage. A light cleaning would be $39 of labor or possibly free if they could do it while you waited. If they didn't open and inspect the Mac, you could ask what that charge would be to be sure it really needs an overhaul and rebuild.

I would say, you could do a surface cleaning yourself as long as the machine is functional and then shop around for a used Mac between now and February. Then at worst, you could then do the $600 service or if the corrosion damages internal components you might be on the hook for the $900 repair. During that time look out for any Black Friday sales and/or new models since repairing it makes more sense if you have AppleCare for several years or really love this particular model Mac. You could also sell it for $700 to $800 to someone that needs a cheap Mac and then pay for the repair once you have an agreement for them to buy it. 

You'd only be out of pocked a few hundred $$ and could buy your choice of new Mac. Lots of options, but the quote is legitimate and you would have a warranted Mac for 90 days post repair should you send it in.