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What do you mean Exposure money? as in their battery powered light?
You can't compare apple and pear, dynamo and battery are two different thing, dynamo aren't cheap, but the fact you won't need to charge the lights anymore, and no need to worry about the battery running out, outweight the cheaper cost of the Exposure.
Look in the dynamo topic, there's a lots more information for you there.
@edscoble @hippy Thanks for the advice. SP look the bomb and I know are highly regarded but cheapest prebuilt wheel I can find with them is £130 from SPA. That's nearly Exposure money for just the wheel. Is it realistic to get a whole setup (including lights) for under that sort of money?