In Fair Trade, fair trade markers are everything while money is only used as a tie-breaker. Consequently, each money left at the end of the game is wasted if it could have been used to win a bid and earn more fair trade markers. In an optimal last turn, you'd place the highest possible trader bid to earn points and use whatever money you have left for the farmer bid to earn tie-breaking money.
However, this is not as easy as it sounds. With only 64 coins, you must bid wisely. Also, since you must pay both the farmer bid and the trader bid before you get any money back, you need more money the last round than your average spend the previous rounds. Let's count backwards to see how much you can spend.
Assuming that you bid 1x as a farmer and 2x as a trader and get back 1x each round, you'll spend 11x the first 11 rounds and Another 3x the last round. This means that you can bid on average 64/14=4.5 coins as a farmer and 9 coins as a trader. Will you ever need the higher bid cards then you might wonder? Yes, because you don't want to bid the same amount each round but bid more when you get more points for the money and less when you get less points for the money.
Let's move on and look at how much money each point is worth. The points awarded differ for different market cards and player counts. For 6 players, the 1 best card awards 18 points in total (3 per player) and the 3 worst 9 points in total (1.5 per player). For simplicity, let's assume that you should expect an average of 2 points per card. This means that for an average round, you pay 4.5 for your farmer bid, 9 for your trader bid, gets back 4.5 from the trader bid paid to your farmer and earn 2 points, i.e. each point has cost you 2.25 coins.
A farmer bid of 10 and a trader bid of 13 for the best market card (6 points to the highest bid) may be a good deal then, assuming that you have the highest bid of both and get back your trader bid.
Of course, this assumes that the other players settle with average bids, but Fair Trade is not about bidding the "right" value but about predicting the other players' bids and outbidding them just enough. If you expect all other players to bid high, you may be better off saving your money this round to be able to bid more the following rounds.
How to predict other players' bids then? One advice is to play conservatively the first few rounds and save money rather than risk wasting them on lost bids. Use those rounds to learn the individual thinking and group thinking. Are players bidding average each round or high one and low the next? Are the cards "locked" in the previous round's market low or high? Has a lot of money left the game or are they still remaining with the players? Those kind of answers will help you determine how much to bid. Remember that you don't want to win every bid (you can't) but rather ensuring that you don't bid too much for what you can expect to get.
Finally the question about who to place the highest bids: the farmer or the trader. Generally, you'll want to make the trader bid your highest each round, since it's the trader bids that earn you points. However, if you're short on cash and expect the other players to bid high, you may want to consider minimizing your trader bid and place a high farmer bid. You'll earn 0 points for the round but hopefully enough cash to earn more points in the future rounds.