Any upgrades that you make early in the game will continue to pay off until you stop playing.  Most upgrades require materials, so the more you upgrade materials, the more materials you have coming in to upgrade more materials, and the cycle keeps going. +2 nails on every contract on 15 contracts per day is and extra 900 nails per month. +5 apples is an extra $275 per orchard run.  Run them 4 times per day will generate and extra $231,000 per month.  Imaging having 2 orchards and level 6 apple books? That's an extra $2,772,000 per month. Or a third orchard? The monetary boost is independent of the orchard level.