If you have all the Temper Passives (and you do want these otherwise improving things is absurdly expensive)
2 Greens
3 Blues
4 Purples
8 Yellows
Per Item
You need to improve each item in sequence, so going to Purple means Green, then Blue, then Purple. A lot of crafters make things Green anyway as we are swimming in them (I have a 800 plus greens for each equipment craft). But this does not apply to all crafters.
It is going Blue and Purple that costs.