Say I bought a one-dot Relic with the Relic merit. Is there any limit set on what the item is? Could the item be very expensive? Or very powerful in its own right (high-powered gun)?
With Storyteller approval, sure…
As the Relic merit states on page 85 of WoD: Reliquaries, "How the character came to own the object is up to the player. The character might have inherited the object, stolen it, purchased it or just found it by sheer luck."
…but one dot is unlikely to get you there.
However, the more powerful or valuable the item is, the more dots it is likely to cost. Relics are meant to be old and strange and laden with history; if the item only costs a single dot, it's likely to have many Costs to use or a Curse of some kind. Taking a "high-powered gun" or a fancy sports car as a one-dot Relic to get around the Resources requirement isn't prohibited by the rules, but it's something I'd look at with heavy scrutiny were I your ST.