Blackmagic seems to be the only company offering reasonably priced HD Capture cards. The problem is that one of them uses PCI-Express and is compatible with Mac; while the other one supports USB 3.0, but is compatible with Windows only. See the following thread for more:
How to capture/record video from HDMI (record game play from XBox 360, PS3 to PC)

p.s I can suggest you an external card that is compatible with Mac OS, but it costs $450+ ...