I can confirm I have the C270 working without power on RPi (and with a wifi dongle also consuming power in the second port).

http://www.amazon.com/Consumer-Electronic-C270-Widescreen-Recording/dp/B00ENF8MP8/ref=sr_1_4?s=electronics&ie=UTF8&qid=1385319232&sr=1-4&keywords=Logitech+Webcam+C270

It's half the price of OP's C615 and the quality is pretty excellent.