Apparently, the Brim Brothers Zone is finally about to come out (I've heard ~$1000). More than I'd personally want to spend, but I love the idea of shoe-based powermeter if I was going to get one. Would work on any bike at any time. Just one, and you're good to go (assuming you don't also want to use it for mtb). As is, there are deals to be had on a use powertap, but you're locked into using a single wheel for whenever you want to do power.