Watermills and silos. "Premium features, premium prices". Okay, lets take a look. Here a medium sized economy, not ready for 2 adventures a day, but quite capable (numbers from my own player level 25-33 economy, until I downsized a bit, to reduce daily maintenance)
- Wheat: 20 level 3 farms -> 20 level 3 silos: 20 * 750 gems: 15000 gems ^= 62.49 EUR for hassle free wheat industry
- Water:
* 1 lvl3 stable, 2 lvl3 breweries, 3 lvl3 bakeries: consumes 11418 water per day (production time + one minute for the ways)
* 1 lvl1 well or watermill produces 360 water per day. For 11418 you need 31.72 level 1 buildings.
* that makes 10 level 3 watermills: 895*10 = 8950: 37.29 EUR for hassle free water
So, we are talking about a "premium price" of 99.78 EUR to make a medium sized economy self-sustaining in respect to wheat and water ... :-)
Calculations based on 24000 gems per 99.99 EUR