How much does it cost to appeal an IRS tax audit and where to start?

The cheapest method is an appeal to the IRS appeals division. This is somewhat informal and doesn't require legal briefs. The next level up is an appeal to Tax Court.

There is no fixed price. It depends on the facts and issues in your case. It often helps to have a tax attorney on your side, though.

Your question is impossible to answer on the facts presented. It all depends on how complex the issue is. I would suggest that you contact a tax attorney or an accountant, explained the situation and how much the claim as and the reason which is being given for it. You can then make a rational decision as to whether or not incurring additional fees is a reasonable business decision.