They will sell Pro Mode "for guitar separately." It only costs a $1 more than the usual $1.99. Not all tracks "will have Pro Guitar mode" however.
"Going forward, all DLC released for Rock Band 3 will be fully Rock Band 3-compatible, which means it'll come with Pro Mode for keyboard and drums, harmony mode, etc. The only thing that's more difficult for us in terms of authoring time is the guitar stuff, so we've decided to sell Pro Mode for guitar separately," said Paul DeGooyer.
"If you're not interested in Pro Mode for guitar, you can still get the DLC that should conform to our current DLC pricing, and you get Pro Mode for all your other instruments."
So what does that all come to? "Right now, a typical stand-alone song for us is a buck ninety-nine. If you wanted to buy Pro Mode for guitar on top of that dollar ninety-nine, it would cost a buck. So it's not going to be five bucks or anything," explained DeGooyer.
"We think it's a really fair price. The alternative would be of course to include it (in the regular download) and up the price for everybody, which I don't think is really fair."
"I should note that not all tracks will have pro guitar mode. (Many bands') guitar parts wouldn't rise to the level that they would need to have pro mode authoring associated with them." Rock Band 3 releases on Xbox 360, PS3, Wii and DS October 26th in the US, and on the 29th in Europe. Are you looking for some Pro Mode strumming action?