I'm confused by the meaning of the sentence, regardless of whether you use "at" or "for".
"The vase was knocked down for £5000" means to me that someone was paid £5000 to knock the vase to the ground! (Similar to "His wife was murdered for £10,000"!)
"The vase was knocked down, at £5000" (note the comma) would mean that the vase was on sale at a reduced price, now £5,000.
If the vase had previously been on sale at a higher price you can say "The vase was knocked down to £5,000".
In order to use "for" I would have to change the sentence to "The vase was on sale for £5,000, a knock-down price".