For some auctions, the cost of 1 bid is 1 credit where as for other the cost of 1 bid can be 2 credits or more (depending on as set by the webmaster). The cost of each bid (1 credit or more) is clearly mentioned on product detail screen under the ‘Bid’ button. The “Credit packs” available are 100, 250, 500, 1000, 2500, 5000 and 10000.