IMHO, Espionage is taking a real huge step backwards in Civ4, and remains the only part of the game about which I am entirely unconvinced. The Civ3 system had promise, and could have been greatly improved upon. For me, the biggest problem with Civ3 espionage wasn't so much the cost, as it was the need to constantly go in and perform espionage on a turn-by-turn, civ-by-civ basis. What I really want is one where you allocate resources to different types of missions and civs, and then let the AI handle it automatically-informing you when-or if-your mission(s) has succeeded, and what the effects of success/failure were.
Yours,
Aussie_Lurker.