Exchange 20 green items (and 5,000 gold) for a blue item, exchange 10 blue items (and 20,000 gold) for 5 purple item pieces, or exchange 3 purple items (and 50,000 gold) for 10 gold item pieces.
After the first exchange the gold cost changes to a crystal cost (20 for green, 30 for blue, 30 for purple).
You cannot exchange scrolls, only items, so if you have a lot of scrolls you don't need then you can sell them for gold. Purple scroll pieces are worth ~2,000 gold each and will quickly net you a nice chunk of cash. Tip by AzakaZero
If you're short on items to exchange, go to the drop-down menu, tap "Items", choose the "Pieces" tab, and check what you have too much of. Anything in the 100-150 range is fair game. Tap an item and choose "Combine". Repeat as necessary and remember, only combine item pieces, not scroll pieces. Tip by AzakaZero
For the purple-to-gold exchange, use Deadly Poison, Necklace of Insight and Bloody Mace. You will likely have more of them than you can use if you've played a while. Opinion by AzakaZero
You should do all three exchanges every day, once each. Paying gold is worth it, paying crystals is not. Opinion by AzakaZero