for jewel, u can get better ones looting when u advanced in the map, currently im getting radiant ones pretty frequently, in nightmare castle defense.

ya, not worth upgrading some jewels, cause their need is so low and the chance of getting it off a dead body in late level is high, and thus the need is much lower than supply.

basically each level up, the cost is increase by about 5, 3 of the same jewel plus some money plus some material for high end jewel. so if there are like 10 levels, then the price of the final jewel should be 5 power of 10 times the price of the lowest jewel, thats about 10millions x the price of the lowest jewel, so it can be like over 100million for the top class, cost of making it.