What's the cheapest way to get into training with power? I used to train with a  stopwatch, and later on, heart-rate. Both required relatively inexpensive tools.

Now I'm interested in training with power (measuring the watts produced) and I'm finding that the tools are at least an order-of-magnitude more expensive.

I was hoping the ANT+ standard would drive down prices a bit.
There is a new website/download/software package called TrainerRoad that will allow you to train with Virtual Power for the price of a Trainer, ANT+ stick, Speed/Cadence sensor and $10/month subscription. 

Checkout the compatible hardware page (linked on the main page) to see if you already have a trainer on their list of known power curves. If not the Kurt Kinetic Road Machine has a very consistent power curve and is available just over $300.

Add an ANT+ stick ($20-$40 online) to plug into your computer, it will pickup the signal from the speed/cadence sensor.

Add a Speed/Cadence sensor (Garmin $25+ - Timex $37+)

Optional (for virtual power) is an ANT+ HR strap

You could start to use the Virtual Power (with nothing but your bike and computer to start with) for under $400, much less if you already have a compatible trainer. The $10 subscription can be canceled anytime and they offer a 30 day money back guarantee, so almost free to try it.