Why in the world would you want to make a sizeable investment like a PowerTap and use your old rim? This strikes me as a really good example of false economics at work. Bite the bullet and buy a new rim, it's worth it.

As regards your question, wheel building prices vary from shop to shop and region to region. I highly recommend that you ask around and find the best local wheelbuilder, and pay that person whatever they charge, regardless of the answers you get here.

MH

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Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
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I'd also recommend getting a new rim. A mavic open pro or cxp-33 shouldn't cost you more than another $60.00. I've gotten several wheels built by the Bike Center in Stockbridge. It's just 20 minutes south of the airport. Their mechanic Jason is really good. He built me a pair of open pros last year and they have yet to come out of true. I paid $35 for labor and $30 for spokes. They even give you a free wheel truing after you ride them for awhile, but i've yet to bring mine back. www.bikecenterusa.com