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.
The older wired Powertap hubs go for pretty cheap on ebay. Here's a listing for $399 for a complete wheel with a Mavic open pro rim:
You'll still need to find a wired Saris head unit though, and it won't work with the fancier new head units like the Garmin Edge 500, but that's pretty cheap.
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.
The expense is usually due to the physical hardware needed.
Somewhere along the way some device needs to measure the power output.
But how?
Well inside the hub seems like the most common version. Thus you need a wheel build around a 'heavier' hub to get this to work, and thus is never cheap.
Polar had a power sensor I never could figure out how it worked, but it had an external sensor along the drive train side back stay. It supposedly watched the chain tension, on top of cadence and wheel speed to calculate power. But that silly sensor was close to $500 or more at the time I looked at it. Still seemed like Voodoo to me, personally.
A friend relied on it, and thought it was pretty good and accurate.
ANT+ is just a data transport protocol, which makes the send/receive module cheaper and more common, but for power, the rest of the sensor is the expensive part.
They're expensive because they use highly-sensitive strain gauges and require careful calibration. A lot of design has to go into working out how to overcome external factors like temperature changes while at the same time producing a light and weatherproof system.
I only have experience with Powertap systems. The cheapest is probably a Powertap Elite+ (£540, cyclepowermeters.com) hub built into a cheap, workmanlike rim (Open Pro, £100?) and coupled with a Garmin Edge 500 computer (£145, Handtec). The Elite+ is heavy but has a stiff steel axle and is ANT+ compatible.
Second-hand systems might be cheaper but watch out -- they can be expensive to repair if dodgy. Common Powertap problems are duff bearings, miscalibration or broken torque tubes (expensive).
There are various indirect systems like the Polar (mentioned in another answer) or the iBike, which works out power from rolling resistance (friction, wind, gradient) and rider weight. They say it works fine; I've never tried it.
The CycleOps PowerCal is a good, inexpensive way to get introduced to power. It uses a derivative of your heart rate to estimate power over a period of time. It also doubles as a heart rate monitor.
http://www.cycleops.com/en/products/power-meters/powercal.html
It is not suitable for short intervals or instantaneous power the way other power meters are. Rather it is good for comparing relative power output over a given period of time (usually a few minutes). It works very well for understanding your power output on long sustained climbs or moderate to long sprints.
There are some excellent reviews of the PowerCal with comparisons to other power meters. It is actually quite accurate when used for its intended purpose.
http://www.dcrainmaker.com/2012/11/cycleops-powercal-in-depth-review.html
My first answer is specific to where you live, I could not mentally justify the big ticket price for a power meter (let alone a garmin as well) but when I found that when stages power meters were coming out with they would be 40% cheaper in US than the would be in the UK/Europe. All this meant I had to do was to be able to afford the power meter, garmin for the data and the book training and racing with a power meter was buy and sell enough power meters to pay for it. This meant I had to trade roughly 20 power meters with a 10% profit margin on each sale to be able to afford my own gear.
So I suppose my answer is you have to find a way to pay for it, whether it means selling your older gear and stock piling the cash slowly and little by little or being a bit entrepreneurial.
