As soon as you start charging for any "feature" of a ROM it's no longer a feature. Once you start doing that there will be a movement to crack it. Net result? People who don't want to pay won't, people who do will reduce their donations by the amount they have to pay, probably more. Overall it might offer a short-term boost to the dev, but in the long run it'll cost them a good portion of their donations, and as a result less income. The Devs missed out when they didn't ask Koush to pay $5/month or whatever to include their ROMs in ROM Manager, so Koush cleaned up on their hard work. I didn't hear any complaints. Still haven't.
Custom apps, yes, I can see charging for them, but why make them ROM-specific? Spread the love, then everyone can have them... And we're back to developing for the Market. Viscious circle. Devs aren't in it for the money. If they were, they wouldn't be doing it, because it's not profitable.