I played Goa last night with a couple of people who were newish to that game, but experienced gamers all around. It reminded me that not everyone understands the huge effect that opportunity cost has in that game.
Bidding is a once-around affair, where each player has a turn to offer up a new tile. The offerer hears the others' bids, and then either accepts the high bid (taking the cash from the bidder) or outbids everyone else and pays his money to the bank. This means that buying your own auctions costs you more than double what the high bidder was willing you pay you.
For example, let’s say you offer a double-ginger plantation and the high bid is 6. If you pay the bank 7, you really just paid 13 (7 real ducats + 6 ducats you passed on). Not only that, but you’ve taken money out of the game economy, inflating the value of the money that’s left in circulation. Since it was your money being siphoned off, that’s probably not a great thing for you.
Bidding on the flag is interesting, because later in the game, not everyone’s actions are worth the same amount of money. If you buy the flag for 5, and you can perform the money action for 6, then at the very worst you can use the free action to make a profit. However, if you put the flag up for auction and the person to your right bid 4, then buying it for 5 in this situation would be a money loser. That doesn’t mean that you shouldn’t buy the flag, but you need to have a very good reason to justify its cost.