Power meters are an expensive option no matter what route you go. The most popular still tend to be the hub and crank based options. Sometimes if you're lucky, you can find a racer looking to get rid of a PowerTap hub on CL because they're upgrading to the newest thing. The easiest option right now if you have a SRAM or Shimano crankset is a Stages (tm) NDS crankarm replacement. They run about $750, and only require swapping out the NDS arm and calibrating to your ANT+ device.
A PowerTap hub can be had for cheaper, but then you also have to figure in the cost of a rim, spokes, cassette, skewer, tube, tire, and build labor. All said and done, it's going to be more expensive and more of a pain in the butt.
I haven't worked with the pedal-based ones, but I assume they're about as simple and effective as the crankarm solution.

Power-based training is effective for athletes looking to maximize performance gains. You can learn a lot about your fitness by analyzing your power data over the course of a ride: Heart Rate vs. cadence vs. power output vs. speed will give you data on your hill climbing, sprinting, and endurance efficiency. Analysis of things like 10s, 30s, 60s, and 5 minute peak power output will give you a much more accurate picture of your fitness gains over time.

For the recreational cyclist with no race-day aspirations, training with a power meter is an expensive waste of time unless you just really like poring over numbers.