If you can afford it, and will use it, it is not a waste of money.
If you are going to spend a few days trying your code on Solaris 10, I would use OpenSolaris forums and other outlets for non-time-sensitive information.
If you have a customer who requires Solaris, it will allow you access to the latest & greatest Security/Recommended patch clusters - a must.
I work on Solaris exclusively, so I have it, and would miss it greatly if it was taken away.
Basically, what benefit would you receive from having it that you cannot do without? If there isn't any, then save your money.
If the benefits are minor, or only a small convenience, it wouldn't make
much sense.
But, if you have means, and $240.00 is trivial to you, then it is just the cost
of doing business.
Hope my rambling thoughts helped.