When First Union Corp. announced it would acquire CoreStates Financial Corp., most eyes understandably focused on the record $17.1 billion price.

But according to Robert B. Wilcox, president of Wilcox & Associates, a New York consulting firm, there's another, equally staggering way to look at this megamerger: He estimated that First Union is paying about $4,000 for each CoreStates customer.

Although no general data are kept on the cost of mergers per customer of the acquired bank, analysts generally agree that First Union's deal sets a record.

This helps to illustrate a big risk that comes with buying a major banking company. CoreStates customers unhappy with how they are treated by their bank's new owner still have plenty of other places to take their money.

"You really aren't buying customers," said Mr. Wilcox, who is a former marketing director for New York's Citibank. "You're buying options to continue doing business with them."

Although a consumer with a checking and a savings account does not tend to generate much revenue for the bank-perhaps $150 a year in fees-banks that fail to keep these customers can suffer considerable damage to their bottom lines.

More importantly, they lose stature in the community, and the ripple effects can eventually reach Wall Street. Wells Fargo & Co.'s purchase of First Interstate Bancorp proved to be a merger that alienated just about everybody-and Wells' once lofty reputation has suffered.

Customer retention is something Wall Street analysts rarely monitor unless something goes really wrong. Consultants like Mr. Wilcox try to ensure that Wall Street has less to fret over.

In recent years, he has advised on how to present mergers to the selling bank's customers in such major in-market mergers as Chemical-Chase Manhattan in New York and large "market extension" deals like NationsBank- Boatmen's.

Currently, Mr. Wilcox is at work on the NationsBank-Barnett merger and First Bank System Inc.'s acquisition of U.S. Bancorp.

Most banking companies start out by estimating that 5% to 10% of an acquired bank's deposits will be lost during an in-market transaction, according to Lehman Brothers analyst Michael Plodwick, but usually a little less in an out-of-market deal.

But whatever the numbers, the unvarnished fact is that nearly every merger is initially unpopular with customers of the selling bank, not to mention local civic and political leaders, Mr. Wilcox observed.

The key to overcoming the disorientation and even anger that people feel about an out-of-town bank's taking over, closing branches, and firing employees is to tell customers as much as possible about how the upcoming merger will affect the costs of their accounts as concisely and clearly as possible.

"It's not enough to say, 'Welcome, we love you'," Mr. Wilcox said. "You have to say that, but you also have to be clear with them about what is going to happen, to frame the customer's expectations. Don't just give them a disclosure book and say, 'Read it.'"

It also helps to delay jacking up fees. When fees differed between Chase Manhattan and Chemical, the merged bank opted for the lower fee. "The bank can raise rates at the industry standard now, and nobody would care," Mr. Wilcox said.

Analysts generally assume that big, experienced acquirers like First Union and NationsBank have the ability to complete mergers without losing too many customers.

But even the best can run into serious problems if they buy banks whose approach to generating and keeping accounts is radically different from their own.

NationsBank absorbed such companies as Maryland National Bank and Boatmen's with barely a hiccup, but it lost a considerable number of customers in absorbing BankSouth, Mr. Wilcox said.

That Atlanta bank, whose acquisition solidified NationsBank's dominance of Georgia, was a heavy discounter of most of its banking products and services. "There was a heavy disconnect" with NationsBank's operating philosophy, which is to charge a higher price for a higher level of service, Mr. Wilcox said.

One thing banking companies have learned is that they can sidestep considerable bad publicity by not offering specific numbers on job losses or branch office closings when they announce acquisitions.

NationsBank, First Union, and National City Corp., to name three, have recently offered analysts and affected communities less clear-cut guidance on such matters when unveiling their latest megamergers. Details are often disclosed later in obscure and arcane filings with federal regulators.

"Banking is a maturing industry," said Mr. Wilcox. "Those kinds of announcements are part of the culture of banking that is going away."

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