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 reduce the taxes you owe - including penalties and interest - at a significant discount. There is no preset bottom limit 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 much less than you presently owe.