I usually tell people $15/acre but prefer to look at the job first, at least on an aerial photo to see what's adjacent to the burn area. We burn some larger acreage tracts for less than $15 per acre for landowners who are on a burn rotation and burn hundreds of acres each year.
The forestry commission charges $20 for understory and $25 for site prep burning (you can find their rates on the AFC website.) This does not include the cost to install firebreaks. I would figure an additional $20-$25 for new fire breaks if you want them nice enough to maintain with a tractor in the future.