There is no real price for upgrades in my opinion. I have paid for people's 15$ upgrades in exchange for 40 diamonds before, and I have agreed to do other's 15$ upgrades for 100 diamonds. It depends on what kind of mood I am in, how much I need diamonds and whether or not they seem like a nice person.
I've seen people get $25 upgrades for 50 diamonds and I agree with you. There's always an approximate value for diamonds:$ but it's just a game so there isn't really a proper value, just value people like