How much resolution/accuracy do you need? Multiply by the number of bits, and you'll get an idea of how many optocouplers you'll need. 8 bits @ 60MHz sampling rate (nyquist requires 2x sampling rate at an absolute minimum)=> 500mbit/sec. There are gigabit class fiber interface modules, but as far as I know, they're a couple hundred USD each. (But then again, I don't especially have any need for one).
Going from analog to digital and back is really roundabout, plus 60MHz A/D converters aren't exactly "quiet". What kind of system is this thing going to be integrated into?