Swindler would interact really weirdly with Wayfarer's cost, especially in multiplayer. Since its cost is defined by the most recently-gained card, you could have a situation where, for example, player A plays Swindler. Player B trashes a copper and receives a curse. Player C trashes a Wayfarer. Since the most recently-gained card is the curse that B received, then C would also receive a curse
The Locusts hex would have a similar interaction, except it would amount to "an Action card costing less than whatever the previous person's gained card cost"
But in some situations, Locusts could actually cause you to get an expensive card. For example, say that I'd just bought a Province, and then I play Vampire, with Locusts being the hex. The person to my left trashes Wayfarer. Now they must get an Action card costing less than $8. Maybe King's Court is in the kingdom, so they're able to get a King's Court! But then also, if multiple people's trashed card is a Locust, then each one's card they gain instead will be cheaper than the previous person ... so if the first person got a $7 card, then the next person must get an action card costing no more than $6 and so on
