DELTA AIR LINES sprung a surprise last week with dramatic news that it was cutting domestic fares by as much as 50% and reducing sharply the price of last minute bookings, a move aimed primarily at the business travel market. The airline wants customers to book via the net. Keeping in line with the rest of the industry, tickets purchased over the telephone from Delta Reservations will now cost $5 more per ticket and $10 more if purchased at Delta ticketing locations, including at the airport. Not always noted for its speed in these matters the airline industry has been quick to respond with American Airlines, the nation's largest carrier, imitating Delta Air Lines' decision. Rivals, Continental Airlines and Northwest Airlines, which also are under assault by rapidly expanding low-cost carriers, took a United Airlines and US Airways more limited approach, matching Delta's cuts only in some markets where they compete head-to-head.