If i were to write a treatment for a music video, and the band had someone else shoot/produce/direct it what would i charge the band? I was thinking 15% of the budget. Any thoughts?

These days, since budgets are so low it's usually best to charge a daily rate.

Most writing jobs are less than 5% of the budget...however they can always fudge the budget so you are short-changed. Daily rate is the way to go. You know the value of your time, stick to it.

How big/successful/popular/rich is that band? And ditto with you :)

It's probably a negotiable deal based on the kinda thing the band want.

The worst job in the world is the 2nd vid for a band that just had a mega hit with their first release. They know everything now.

Tread carefully 50% up front.

Cheers.