How to earn Money
Sell all you enhancement items... They are worth more in gold than junk items.
Don't sell your junk items, instead use them for upgrades.
How to upgrade using less money
Upgrade junk items using 6 other junk items, this usually upgrades an item to lvl 7.
Depending on the number of stars it gives a higher exp on upgrading.
upgrade 6 cheap items to lvl 11 or higher.
then upgrade your lvl1 5 star item using your high lvl junk items.
and you got your self a max level 5 star item for less money.
I hope this helps.