What you are getting is a more proven, tested, solid OS. Solaris and Oracle can simply do things that MySQL on Linux can not do. The commercial Unix varieties are offered by companies with hundreds of engineers with lots of experience building their OS. The free varieties don't have that. Therefore the commerical Unix vendors provide features free Unix doesn't have, for example Solaris has dtrace, and the next version will have ZFS for managing your filesystems. No version of Linux has tools comparable to those. And typically commercial releases are more stable due to much more work being put into the release (and more testing). Paying all those engineers to work on it full time means you get a lot more effort being put into it then relying on volunteers.

Also the point Neo made is very true. You are paying for support also when you buy that $65,000 OS. If my Solaris box crashes at 3 in the morning I can call Sun and if the support staff can't figure it out immediately they'll have a kernel engineer talking to me within an hour or two helping troubleshoot it. As an example of the kind of support you get, I remember one instance where our E10K (Sun's top of the line circa 2001) had a hardware problem. A Sun hardware engineer showed up at our office within 2 hours and he had about a quarter million dollars worth of spare parts couriered to us at the same time so that no matter what the problem turned out to be he could replace it without having to wait for parts to be delivered. You can't get anywhere near the level of support a commercial Unix vendor gives you with free versions. You can pay Redhat or another linux company for support if you want, but then you end up paying just as much. And to be honest, Sun, IBM or HP probably have more OS engineers than Redhat has total employees - which do you think will be better able to support critical systems when there are problems?

THAT is what you are paying for when you buy commerical Unix.