This one is going to cost a lot of money. The later weapons, like the Revolver or Shotgun, all cost $5000+. I did this one after I got the jackpot achievement, so I had tons of money for guns, but there should be enough money strewn throughout the game for you to grab this one without doing that.
There are something like 12 weapons in the game, so 8 isn't even all of them. You can just pick the ones that you like and you'll be fine.