People Express and its new acquisition, Frontier Airlines, will offer connecting flights between 83 American cities for unrestricted fares ranging from $99 to $199, undercutting the competition by up to 70% . . . Security Pacific Corp. and Sumitomo Trust & Banking Co. of Japan have agreed to found a joint-venture firm offering investment advisory and management services in both countries . . . American Express said its board authorized the repurchase of as many as 10 million shares of its common stock on the open market. Also, the firm said it will make a $100-million capital contribution to its Shearson Lehman Bros. unit.