Probably the cheapest is diametrically opposite to the best.
You probably should place bids at "the big four" dropcatchers to be have any reasonable chance at the domain. They are Pool/SnapNames/NameWinner/ClubDrop.
You must place a bid at the expiring domain's registrar's preferred auction partner.
Price is irrelevant until the dropcatcher catches the domain. You need to have a bid in at that dropcatcher to stand any chance at all.
While you won't catch the most premium names, the CHEAPEST backorder service I know of is a WWD reseller account. I so sell these, so if this is considered advertising please remove this post and accept my appologies.