My issue, which makes the Wishing Well a bit useless, is not so much the value, but the frequency. The previous Wishing Well I set up had given diamonds 3 times in its lifetime. The rest of the time it gave miserly amounts of goods, coins, or supplies. I might still have one in my inventory, not sure as it's inconsequential. 30 diamonds in 100 days wasn't worth much, since I use diamonds (rarely) to add an expansion. If the value increases to 25 and I get diamonds 3 times in 100 days, it's still rather useless. Others have reported better frequency, but whenever an RNG (Random Number Generator) is involved I don't seem to have very good luck.