I don't think the price is that bad for a commerial license. Yes, it's from an open source but he's actively working on it and improving it. If I'm paying a programmer to develop my site, why would I want to pay someone $50-100/hr creating and supporing something similar to Highslide?
I can work on it as well using ext2, but I'd rather focus my efforts on the core functionalities of my site.
Just my 2 cents (or whatever currency you use)