As with all trades, the traders should get something better than what they are trading. When I say better, I mean something they want. Not something mathematically better/more expensive, but simply what they want.
The total worth of the trade is a value judgment. Would I trade coturnix for buttons? No, because I don't want buttons. Would I trade coturnix eggs for buttons/button eggs...maybe. Would I trade my entire coturnix flock for an ostrich, mule, horse, milking cow...maybe, and I might me a bit up mathematically, but sense I don't want, or really want to care for the traded items, then I wouldn't trade.
Brass tacks! A fair trade is when both parties get what they want!