It kinda doesn't make how I have 500 green tempers and only have to use 2 for a 100% upgrade, in my opinion at least.
I know it's not that big of a deal but it just doesn't really make sense; so my suggestion is to increase the amount of lower tier tempers needed to achieve 100% upgrade ratio. Don't mess with the drop rates or try changing their market prices (not possible anyway or people will riot) - just increase the amount cos sitting on 500 green tempers and 5 gold ones doesn't make much sense.
Would be a good gold-sink opportunity as well, we know how some people are sitting on houndreds of millions of gold anyway.