It's not that Hogback knew the Strawhat's bouties from the news or anysuch thing. He had the little ghost man sneak through the ship collecting data on their new "prey."
For one, Im not sure why the SH's would have written evidence of their bounties on the ship.
Two, if they turn them in for bounty they must have contact with marines or some form of athority that pays out. I don't see why the person turning in the pirates (maybe absolem when he's not invisable) doesnt collect bounty posters for refernce.
The fact that they have such high bounties doesn't make them excited because of the amount of money they are going to make. Rather, they are thinking that their going to have to pull out all the stops compared their usual fare of every day pirates.
The Bounty's have a good amount of sway in their thinking, shown in the fact that HogBack won't let Asolem touch nami because of her bounty. Though they are getting reared up for a good show because they know the Sh's are undoubtly strong and the norm won't be enough this time.
(which I props them, for I belive they are the fisrt bad guys not to underestimate the SH's)
Pirate hunters don't call their target's "prey," as Absalom and the scary forest crowd referred to them. They have much more nefarious plans than simply turning them in for bounties.
Pirate hunters call their bounty's prey all the time. As for their plans, most likely they will try to do more then just turn them in, but the money is still at the end of the deal and the whole part of being a pirate hunter.