How you set the cost of an item depends on two things:

1.  The TYPE of item:  Purchase, Raffle draw, or Auction

2.  The coin economy settings

If you want to change the rate at which users are making purchases, you can change the coin earning economy and/or the cost of the item.  We recommend you change one of those at a time and measure the effect.

When listing a new item, consider the average number of coins being earned per user per day.  Click on the blue graph icon next to the Cost field:

For Purchase items, consider making the cost relative to the real value of the item and/or the amount of time it would take a user to earn enough coins to make a purchase.

For Raffle draws, the cost is the number of coins the user will spend to make an entry to the draw.  Consider making the cost low to encourage users to keep purchasing more entries.

For Auctions, set the opening bid cost relative to the real value of the item yet low enough to encourage users to participate.

There is no perfect formula for managing the economy.  You'll likely want to try things out and make changes over time.