Yes, snipers do effect the final price of an item... sometimes. For hot items, it doesn't matter because people bid regardless and a sniper will drive up the price at the end.
But on an auction for an item not really wanted, a sniper does effect the price. The current high bidder probably will just bid the minimum required just to win thinking that that no one else wants it and a sniper will take it right from under them at auction end.
So, pretty much the effect a sniper has on an item depends on the demand.