This is a gas burner to fit into a Trangia hiking stove. It boils water about 3-fold quicker than a meths burner.
It was reduced on a Lightning Deal to £16.79 including delivery (without Prime) on Dec 27th. It looks like they only had 9 in stock, as the percentage left varied by 11% each time.
The price has now gone back to £23.99, but you might find it returns as a Lightning Deal before too long.
The official thing from Trangia is £44.50. The reviews for this one seem OK.