Since you seem to consider buying on ebay to qualify as available, I'd nominate the TI 34010 and 34020. Looks like you can pick them up for around $50-60 apiece (though quantity available may be open to question--most seemed to be lots of one).
There seem to be at least a few Intel 82876's available as well (for around the same price).
Direct from the SNES development manual:
As for how it's possible: game consoles are almost always sold at a loss. Vendors make their money off the games (which is why they work so hard to assure that nobody but themselves can publish games for their console).