Well the question is rather simple, how much is every type of res worth in your eyes?
The game suggests that metal and crystal is equally worth, personally i think that crystals are worth much more than metal ^^
I know that is of course influeced by your needs and the supply, but if you think of a game where you get an equal amount of res is wich ratio would you want them to get?
So just give us the ratio you think is alright for the three

Persoanlly i would choose:
16:2:3 Credits:Metal:Crystal
So whats your favourite ratio?