why bother. ;) let them set the prices.
from: http://www.stockmusicsite.com/stockmusic/alliance/default.cfm
3. YOU ARE IN CONTROL - You retain full ownership of your content. You choose the price to sell at. You configure your tracks with title, description, keywords, etc. You can sell your tracks individually or in multi-track "soundpacks". You can configure one or more artists under your account if you have content from multiple different sources. We make it easy with batch upload and batch edit capabilities, a price administration wizard and other helpful tools to maximize your control and minimize the time it takes to configure your tracks just the way you like. You can remove or replace any tracks at any time.
you can however, offer the minumum and maximum rates and even a guideline rate. and for these you should do a comparison shop and see what others are doing.
one example (same site as link above):
http://www.stockmusicsite.com/client/options/pricing.cfm
do however, check other sites as well.
http://www.google.com/?q=royalty+free+music