(MONEY Magazine) – The very model of a modern airline, superprofitable Southwest built its 23- year success with low daily fares on every flight -- few frills, no Saturday night stay, no three-week advance-purchase requirements. Now, upstarts like Kiwi (based in Newark), MarkAir (out of Anchorage) and Reno (in Nevada) are joining the party. Most of them are expanding routes and driving down fares by a sharp 35% to 70% in every market they enter (right). "These small, low-cost carriers are changing the way airlines do business," reports Larry Crawford at Avitas, a Reston, Va. aviation consulting firm. Ready for travel, budget style? Then prepare for some unusual twists. Prime among them: Some upstart carriers do not pay the required fees to be listed on the four major computer reservations systems used by travel agents. "Most agents will still be familiar with local low-cost carriers," says Stephanie Kenyon at the American Society of Travel Agents, "but they may tack on a $5 to $20 ticket fee for the extra time." To snare the lowest price, you're best off calling the carrier yourself. At left is a cross-country sampling of the current round-trip fares introduced by the upstarts (sometimes other airlines match these fares). Amounts shown alongside are the standard 14-day advance-purchase price for that route at press time.

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