Fyi, you can buy an entire wheel with a Sanyo dynamo hub which several posters here have said work well (it's drawback is that it's a little lower quality, and turning it off doesn't actually reduce the very small amount of dynamo resistance from the hub) for $135, plus $5-$15 in shipping according to a quick search -
http://www.calhouncycle.com/productc...wcart.asp?cs=1

I just mention it because you mentioned "dynamo" in the title. Biggest drawback would obviously be cost, as one would also need a light to go with it, which would cost around $110 for a good one.

The cheapest route is to buy 2 cheap but high lumen output lights off of amazon. Carry both, if 1 fails, use the 2nd one.

I like the Phillips Saferide as it has a shaped beam, but it's more around $150 total. It's the light I most recommend to friends and family though, best light for road biking I've found so far.