Remodel Watchtower into Catacombs, gaining Catacombs, then revealing Watchtower to trash Catacombs and gain a card cheaper than it. Not sure if it counts since the second one isn't immediately after the Remodel, it's after the first card was gained.
[EDIT] Dang it, got Sarnath'd twice in a row. I had the stonemason case typed up, then I saw the post and changed it...
List of possible cards for gaining a card costing (strictly) less than another:
Catacombs
Border Village
Stonemason
Haggler? Only on buy, but maybe Black Market could work here. Also, technically you have to buy then gain, so it's not immediate.
Develop? Must be one less, so it specifies something else
Saboteur? Must be at least 2 less, so it specifies something else
Don't think I've missed any here, so these are the only cards that specifically mention gaining a card of lower cost than another card.