All "can run on" are taking into account what it run
well on. All $ is in USD.
Windows XP - Costs about $30 for a used Home Edition | Windows 7 - Costs about $90 for Home Premium (7 machines should have around 2GB to run well)
Desktop - Windows | Can run well on virtually any machine in the last 7 years, and you can build your own. Costs $150-$400 for a good machine |
Laptop/Notebook - Windows | Can run on virtually any machine in the last 6 years. Customizable, to a degree. Costs about $200-400 for a good machine |
Netbook - Windows (if 7, probably starter edition) | Can run on virtually any machine in the last 4 years or so. Customizable, to a degree. Costs about $150-300 for a good machine. |
Tablet - Windows XP | Good luck finding one. May cost just around your limit. Probably not customizable. |
Mac OS (costs around $30 for Snow Leopard, and then another $30 for Lion)
A desktop is basically your only choice for a Hackintosh route. You'll almost definitely have to build this. Costs about... eh, $400-500 for an average Hackintosh. Fairly customizable.
Linux (costs... nothing! [in the vast majority of cases])
Not much to say here. The stats are almost in sync with Windows. Use that table.
Chrome OS - Not available for custom installation
Your only choice is laptop. That will range from $350-500.
Chromium OS - Free of charge
Use the Windows table as a general guide. Be warned, your hardware here is restricted.
iOS - Not available for custom installation (correct me if I'm wrong)
Obviously, you're limited to the iPhone, iPod Touch, and iPad. A blanket cost for these can be from $160-your maximum
Android - Technically free of charge, but not really worth the trouble of custom installation
You have many choices here. Phones, of course, have 2-year contracts at advertised prices Anyway, you have many choices here, ranging from $0-250 for a phone, and $200-your maximum for a tablet.
XMBC and Boxee - Need to run on Windows in some cases, but don't need to be super-powerful for their job.