You're going to be VERY hard pressed to find a good HO alternator in that price range.... at least for a new one.... You can find all kinds out there on the internet that claim they're HO but they either NEVER get to the rated output or their output at ur idle RPM is worse than your stock unit....
If you're just running the equipment in your sig then you shouldn't need a HO alty.... upgrade to 1/0 and do the BIG 3 in 1/0 and you should be doing just fine without the HO alty....
That would be a good place to start..... the BIG 3 is the first thing I recommend to the up and coming car audio guys around here... it's cheap, easy to do, and will have your electrical ready for the future when you go to upgrade to bigger and better things....