At this time I only know of three different power units that are worth using.

1 SRM power cranks- very expensive at $1770 (all prices in USD)for the base unit. I've seen used systems go for $750 and up.

2 PowerTap - also expensive at around $700 to $900 for a complete system with software. They now make two models, the standard and the Pro with the Pro being about 200 dollars more. Used they can be had for under $500 on Ebay but make sure it's the current Graber built unit and includes link software.

3 Polar 720 with power attachment - Also expensive at around $300 for the HRM and $300 for the Power unit. You can buy it piece by piece which is nice if your budget is limited.

I'm currently running a standard PowerTap set-up on my training bike and like the features it offers. With that being said, I only use it on my solo training days when I'm doing wattage specific training. I very rarely use it when riding with groups as in that application it seems like more of a toy than anything.

Unless you're very serious about your training and have either a detailed plan or a coach to plan your training for you, it may not be worth the money.

Good Luck