I've been involved with controlled burning for 37 years, and I can say that times have certainly changed. In the seventies, in this part of GA (middle) and especially south of here, you couldn't drive around in late winter or early spring without smelling or seeing smoke, and everyone was used to it. Thousands of acres were burned every year. Back then the main concern was fire getting out and doing physical damage to adjoining property. Now, smoke is the big issue.
We used to burn in late evening and at night, but that's taboo now due to smoke settling on the highways. Burn blocks were 500-1000 acres, and we set all of it on fire at one time. Of course, it took days to burn that much area with a backfire, but it didn't matter since the public knew what was going on.
Now we have purely ignorant folks living in the country who don't have a clue why anyone would want to set fire on purpose to the woods. Drivers refuse to slow down for smoke, and, quite frankly, the liability potential is so great that my burning has been cut way back to a fraction of what it once was.
My advice is to get an insurance policy that covers damage to adjoining landowners as well as your client, make a "burn plan" which includes weather info., smoke management considerations, etc., insist on wide, permanent type firebreaks if at all possible, and get certified in the state you're in if such a program exists. The more credible you appear to a jury and the more paperwork you have prepared in advance, the less likely you'll lose everything you own if you have a problem and are involved in a court case.
As for charges, mine vary widely depending upon things like fuel buildup, cover type, condition of firebreaks and number of cross breaks, adjoining properties, downwind development (or lack thereof), rural, semi-rural or semi-urban neighborhood, local forestry commission politics, etc., etc. I will charge a much lower price for burning in a "safe" area on land belonging to a client that I made or will make a timber sale for or long-term clients that give me regular business. One time jobs cost more since there's less opportunity for profits in the long run. Understory burning is generally less dangerous than site prep burning, all things considered, and my charges for burning only can vary from a low of $5-6/acre to $20/ac. not inc. the cost of firebreaks. I just quoted a job that came out close to $45/ac. which included permanent firebreak construction and one time burning. The burning alone was $17/ac., and the site was relatively well located for burning. Tract size plays a big role also. The larger the area, the less I charge, in most cases. I don't want to be vague, but fees for burning must be assessed on an individual site basis. Remember, some tracts are simply too risky to burn at any price, not from the standpoint of fire containment, but due to smoke management concerns.
Ironically, the less prescribed burning is done, the more dangerous it becomes with heavy fuel accumulations, potential for tremendous amounts of smoke, and even mortality in the stands of timber. Prescribed burning is essential for southern pine management, but it's quickly losing ground to political factors and ignorance on the part of the public. Smokey did too good a job !