That's true, most games will go down or stay the same price over time, unusual for games to increase in price. The rare games tend to flucuate since there are so few sales.
A seller couldn't camp the offer, once the game leave "In Queue" status and hits "Waiting for Confirmation" status, the price is locked in. Only way to change the price would be to cancel the trade. If the seller cancels their offer and re-lists the game trying to get a different price, they will lose their spot in the queue. Like others have said, since games usualy go down in price if a seller gets matched, they will try to confirm it as soon as possible so they can send it out and get the credit before the game goes down further in price.
Angel, in your case, you'll get $22.50 for the game, since you were matched up before the price change. That is an unusual case, and it should be rare for the price to increase after you were matched up.