This setting decides at which price your coins will be converted into your selected currency.

Counterpart Prices:
The counterpart price of your trade will be taken.
When you buy 1 BTC for 500 USD, we will convert 500 USD into your selected currency according to the trade price.
This setting is realistic, as it contains the spread and the fee. That's why your "Cost per Unit" will be slightly higher than the "Sell Price per Unit".
Shown in BTC, the above example may result in something like: Buy 1 BTC for 1.02 BTC (assuming 2% spread or fee).
Only if you did a perfect trade, without any spread and fee (what's nearly impossible), both values will be equal.

Transaction Prices:
The transaction price of your trade will be taken.
When you buy 1 BTC for 500 USD, we will convert 1 BTC into your selected currency according to the trade price.
This setting contains only the actual transaction price. Spreads, fees and other additional costs will be ignored. Your "Cost per Unit" will be exactly the same as the "Sell Price per Unit".
Shown in BTC, the above example will result exactly in: Buy 1 BTC for 1 BTC.

Best Prices: (recommended)
The currency with the lowest conversion difference (spread) is used in this order:
1. Fiat currencies (USD, EUR, GBP...)
2. Bitcoins (BTC)
3. Altcoins and commodities (ETH, LTC, XAG...)