Effective from March 15 - four days after Christie's new structure comes into force - Sotheby's will charge 25% on the first $100,000/£50,000 of hammer price, 20% on the portion of hammer price above $100,000/£50,000 up to and including $2m/£1m; and 12% on any remaining amount above $2m/£1m.
With exchange rates as they are, it means that buyers in the US paying between $80,000 and $1.6m hammer will pay a higher rate of premium than at Sotheby's sales in London.
The move, announced in Sotheby's latest set of results, marks the first significant divergence between the two rival auctions houses in buyer's premiums in recent years.
The results also showed that Sotheby's have opted for a short-term hit in costs to improve their long-term liquidity, credit facility and financing costs.
Global Sales Totals
This move to safeguard the company's future and raise the stakes in competing for consignments comes at a time when, although narrowing the gap with Christie's, Sotheby's remained 14% behind in global sales totals for 2012, at $5.4bn (£3.38bn) compared to the $6.27bn (£3.92bn) for Christie's.
Overall profits were down $63.1m (37%) at $108.3m.
The increasing importance of private sales was shown in their 11% rise to $906.5m - matching the 17% share of all revenues in 2011. Associated commissions were up $6.8m (10%). Meanwhile finance revenues, associated with loans and other facilities, rose by $5.7m (47%) and, at the year end, Sotheby's finance segment loan portfolio balance was $425.1m, almost double what it had been the year before.
Whether this is linked to the record level of sales from single-owner collections is not clear, but that record comes at a time when auction revenues as a whole fell by $79.5m (9.4%) to $717.2m in the wake of a 10% decline in net auction sales.
It also comes as disciplines become more polarised. November 13 saw the highest auction total in the company's history when the New York Contemporary art evening sale took $375.2m, while the firm also registered their highest-ever annual totals for jewellery and watches worldwide.
The focus of change appears to underscore what chief executive Bill Ruprecht called the "very competitive climate for high-end consignments". He acknowledged the "stiff comparison" to 2011, "one of the best years in Sotheby's history", indicating that new markets in China and Brazil, as well as further honing of client services, would be priorities.
Significantly, he also talked in detail about the company's online offer, something that has been noticeably missing in previous reports.
"We are pushing innovations on our web platform to make art more accessible and to reward our best clients," he said. "We offer our clients personalised account information in the language of their choice, we're the only auction house to provide digital catalogues and a magazine in downloadable format, and the only auction house with a Catalogue App program that offers twirling 360 degree views, note taking, real-time sale results and video."