The IRS Code states, "We will accept an Offer in Compromise when it is unlikely that we can collect the full amount owed and the amount you offer reasonably reflects the collection potential..." (Internal Revenue Code section 7122).
Often it is possible to fully and completely eliminate the taxes you owe - including all penalties and interest - at an enormous discount. There is no preset minimum that the IRS will accept to settle your debt especially if your offer is done "right."
If done correctly, your debt may be settled for only 5-15% of what you presently owe. The key is to determine the least amount that the IRS will accept from you before you make the offer. This may be difficult to determine on your own and in this case, retaining a CPA to act as your advocate ensures that you make a confident and informed decision when dealing with the IRS.