I had to try it for myself. I used Bidcactus (you bid on giftcards). You pay $0.75 per bid.
After buying $375 in bids and then biding. I won $150 in gift cards. Plus they charge you $15 for shipping for each gift card that you won. All my gift cards came in one envolope, but I still had to pay $15 per card.
I seen a $250 Wal-mart gift card sell for $28.97, that is 2897 bids at $0.75 per bid which equals $2156.25. Now there are a bunch of people that bid it up that high. The odds of you only makeing 5 bids to win this item is slim to none.
You are not going to win enough auctions to counter balance the amount that you spend. Everytime that somebody bids it adds 15-20sec to the bidding time. So its not like Ebay where you can just drop a last second bid and win the auction.
Take my word on these penny auctions and Stay away. Like I said, I had to try it for myself to see. Yes the ones that I did win where at great prices, but you lose on a bunch of auctions too.