Correspondence with developer:
Q: How does Charles Ross, Esq. work? The card doesn't explicitly states its time window. Can you use its ability then in the next round use up the cost reduction? Maybe even use it twice before playing an item? It using at your location, does it mean your location when you use the ability, or when you play the item (if you use the ability and then move, where can you play an item at reduced cost)?
A: Correct; Charles Ross’s ability applies until it is used, so if you use it in one round you will still get the discount next round, and if you use him more than once before playing a card, you would get a –2 discount. As for your second question, if you move after using Charles Ross’s ability, the discount would apply to the next Item asset played by an investigator at your new location, not the former location.
So basically you can get one additional resource from it per round. Awesome for a 2 resource card!