In response to a quandary regarding how to evaluate one's Maiser considering the rising inflation Reb Moshe Feinstein eloquently explains how inflation is to affect one's giving of Maiser. The following is a short synopsis of the Teshuva:
Maiser is calculated from the real profit rather than the nominal profit earned. To illustrate, suppose Mr. A Purchased An Asset For $1,000 and sold it two years later for $2,000. Suppose further that the inflation rate in this interim period was 100%. Considering the 100% inflation rate, the nominal profit of 100% on the sale is reduced in real terms to zero. Consequently, the nominal profit earned here would not be subject to any Maiser obligation. R. Feinstein further posits that the difference in the purchasing power of the monetary unit in the relevant periods of time only considers changes in the prices of necessities. Changes in the price of residential homes and luxuries, however, do not enter the index.
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