Unfortunately Rickenbacker is VERY protective of their designs, and shuts down quickly any company that tries to make copies of their instruments.
Daisy Rock's got a really nice semi-hollow electric 12 out, sounds great and very inexpensive. And it's white and not at all girly-looking like some of their axes.
Fender makes a Strat XII - it's up there in price, but not as bad as a Rick. They also for a time made a 12-string version of their Squier Venus (the infamous Courtney Love model) that you might look for used.
Yamaha, IIRC, makes a Pacifica 12-string that's inexpensive.
Other used ones to look out for - Peavey's Milestone 12, and Danelectro's 12-string from a few years ago.
There's a guy on eBay who regularly sells a solidbody electric 12 called a Saratoga for around $259 US + shipping:
http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&
category=2384&item=73127... 8&rd=1
Other than that, you might want to consider building one from parts. Warmoth I know makes 12-string necks - get a body, electronics and other hardware and go to town.