The amount of coins depends on the number of sent collection items:

- when sending 1 item, you receive only coins for this item;
when sending 2 or more items, you get coins for each of the items and a BONUS (red icon with numbers) for each item sent.
For example: 
If the collection item costs 50 coins, and the bonus is 25 coins, then for sending this collection you will receive 200 coins (3 items = 50 coins + 50 coins + 25 coins + 50 coins + 25 coins = 200 coins)

Collect and exchange more collectibles, and you will quickly replenish your stocks of coins.