Originally Posted by WhyFi

If your crankset is a Hollowgram 2.0, you can get a Stages/4iiii/Pioneer left crank arm for it in the $360-400 region - it'll likely be a 105 or Ultegra arm, but it'll fit (again, provided your crankset is a Hollowgram). Watteam PowerBeat can be a little more affordable, but the install is more complicated (again, pending compatibility). Short of those options, a Power2Max crankset can often be had for the $500-600 region depending upon your bottom bracket and existing chainrings. The very easiest option, and no bones about it compatible, would be a pedal-based power meter, either single- or dual-sided. The main drawback to these is that your cleat options will be limited.