I hope Nintendo is smart enough to realize that they can't charge as much for an NES game as they can for an N64 game. If we look at Apple's 99 cents for a 4-5 MB download we get a pretty good idea of what it costs per MB to deliver content. NES games are WAY smaller than this, the cost would be almost nothing. I'd guess at the kind of volume they're likely to be doing its way way less than a penny a download in cost for your average NES ROM.
So what should they charge? Think of it this way: if they charge $5 each for NES ROMs, I'm going to buy only two or three of them. If its a buck, I have maybe 10-15 of them I'd probably get. If its a quarter, I'd probably buy hundreds. Since the cost per unit is practically nothing, they'll make a lot more money off of me at a quarter each.
I don't know if my likely spending habits are the same as other people here... so I don't know what's the "sweet spot" for them, but that pricing is a major issue, and it could be the difference between the backwards compatibility being a major selling point and a train wreck.
Here's what I'm picturing. Go into Circuit City, or Best Buy, or wherever, plop down $20 for a "Revolution Download Card" that has a scratch-off code you enter in (like a calling card). Then you can spend it however you want on content. Maybe they could even have pack-in cards that come with some of the new first party Revolution titles (or even other stuff like cereal or potato chips) in smaller denominations to get people to try out the service without paying for it. Maybe you could even win some credits playing online titles. $0.25 for an NES title, $0.99 for an SNES title, and $5 for an N64 title.