Graduate student Orson Calderon savored his last Big Mac attack this week, saying an imminent 50 percent increase in the national sales tax would crimp his budget.

"I can't afford McDonald's any longer," said Calderon, 24, who finds the 20-peso ($2.90) price of a sandwich-drink-and-fry combo less appealing now than when it cost about 16 pesos. A couple of tacos and a soft drink from a street stand costs 8 pesos.NAFTA helped open Mexico to a flood of American and U.S.-style consumer goods. But the peso's recent slide against the dollar has made imported goods - from fast food to American CDs - too costly for many Mexicans to enjoy.

The peso, worth 29 cents before the Dec. 20 devaluation, is worth about 14 cents now, so some prices have nearly doubled on imports. Overall, inflation is expected to rise 40 percent in 1995. The minimum wage in Mexico is 16.30 pesos a day ($2.40).

"There's no way I can buy CDs like before. Everyone I know is cutting way back," said Alfonso Martinez, 19.

Since Mexico began chopping its tariffs and opening its markets, imports from gin to blue jeans have been flooding in. Even frozen tacos are imported from the United States.

NAFTA, short for North American Free Trade Agreement, accelerated this trend by creating a trade zone with Canada and the United States.

But in opening its markets, Mexico racked up a $28 billion trade and services deficit in 1994 - the imbalance between what it imports and exports. That was a major factor in the peso's slide.

With the devaluation, imported goods have become more expensive, and the trend toward imports has been rapidly reversed.

As a result of the economic crisis, prices of public transport, electricity, gasoline and all but a handful of basic foods have risen. On Saturday, the general sales tax is to rise from 10 percent to 15 percent.