Here are some ideas I thought of to revolutionize the prep cost.
1. The entire kingdom could gain "XP". The XP could be a combination of XP gained by heros and the total level of upgraded buildings etc... As it gains levels players would receive preparation slots, and a preparation allowance.
At first you can only select 1 prep. As you get more of an allowance you can select "bigger" options, but still only one at a time. Untill you can select two etc...
I'm leaning towards this option myself. The current model would fit right in with an allowance mechanism, whether or not the slot restriction also applies. After all not all preparations are equal.
Of course, more quests/slight rewards for low-preparation and no-preparation runs would be nice. Maybe even quests that requires certain preparations or combinations of preparations. The end-goal should always be adding more variety to the game.