As the cost of borrowing has declined the past 90 days at West Coast savings institutions, so have their deposits and other debt.

The average cost of funds index was 4.299% during March, according to the Federal Home Loan Bank of San Francisco Monday. COFI, as it is commonly known, fell from 4.376% the prior month but was higher than 3.624% reported a year ago.

The index, which is reported on a one-month lag, has declined each of the last three months.

COFI is calculated using the interest expenses for 11th District FHLB member savings institutions based in Arizona, California, and Nevada. Those expenses include deposit accounts, FHLB advances and other borrowings. During the latest month, $463.7 billion average total funds were used in the calculation -- the fourth consecutive monthly decline.

COFI is used as an index for adjustable-rate mortgages -- which accounted for 18% of total applications taken in the latest weekly Mortgage Bankers Association survey.

One competing ARM index, the 1-year Treasury, yielded 4.91% Friday -- unchanged from a month earlier, according to Federal Reserve data.

Another competing index, the London Interbank Offered Rate, or LIBOR, yielded 5.3581% in April, climbing from 5.3212 % in March. As with the COFI, LIBOR had decreased from February to March.