Okay, this might not be what you are looking for, it's a little spendier than then the items listed above. I think the item that is a real steal is Daern's Instant Fortress. Let me see, I'm a high and powerful mage looking for a place to settle down and teach a few apprentices. I see this beautiful 3 story tower in an idyllic setting and question my DM, "How much for the tower?". A few flips through the DMG, hmmm... 50,000 gp. I spend then next couple days pondering whether to purchase the tower or not when a wizard shows me an interesting little trinket. A small metal cube one carries around that can expand into the same 3 story tower. With the added portability, add on adamantine walls and wizard locked doors. Maybe a few other bells and whistles (my books are packed up so I can't look for certain). How much for this wonderful artifact? 55,000 gp.
That's a pretty mighty upgrade for 5,000 gp. Sold.