I'm pretty sure passionate outbursts of a slightly exxagerated nature don't hit the oath rod, because they typically believe it at the time they said it.

Also, it could easily be true, or thought to be true. It's undoubtedly a one of a kind antique, so cash value might be enormous, I'm sure the Queen of England owns quite a lot of crap that's worth wads of cash, and Elaida holds a similiar level of wealth and importance. How much does the Mona Lisa go for? This is a women who rules a place that can both afford expensive antiques and would almost certainly have them. That rug could easily be worth more than the average village.

But the straight cash value aspect isn't necessary. Elaida believes Emond's field is the place that spawned three troublesome ta'veren and two very powerful channelers who oppose her, so she probably thinks the village is worth a match and a can of gasoline.