That's a healthy budget for what you are seeking.
IMO craigslist is the way to go (check Sacramento? and any close cities) constantly check not only bikes by owners but also in general.
Check on Sunday and due to the holiday, Monday (start checking ads now to get comfortable with the site)
If you even THINK it's a good deal and might work, DO NOT HESITATE and respond to the ad ASAP (great deals where I live can go in minutes...
You are probably looking for a MTB around 17" (frame size) but that's just a ballpark and a test ride will be necessary.
I live in a campus community and colorful simple/single speed fixies are real popular for the riding you describe.
Reserve some of your budget for a good lock (it's terrible where I live) and if the bike has quick release saddle/wheels, consider swapping those out to non quick release (deters theft, expecially of your saddle!)...
Good luck - I see simple, older, well built 90's cro-mo framed MTBs all the time for under $100 (it just takes the commitment of a day or two watching craigs....)
But seriously, IF you see a bike just posted that appears to be a great deal - DO not hesitate and reply immediately... you can reply and then google/bikepedia the bike after... and then change your mind....
also you will see a bunch of $200 bikes that were only $369/most likely $299 on sale when bought... personally $100 would be tops of what I would pay for such a bike if it was in terrific condition and perfect for the rider...
Here's an example of a reasonable fixie (might be too tall for you and definitely a lil far...lol)
Fix-D Magna Flip Flop Single Speed Bike
Here's a beautiful, light, older (aluminum) cannondale that I had to sell recently (just too tall for me)
If I ever run across this model again in a smaller frame, it will be my go to commuter.
Bought at a garage sale, cleaned up (not bad to begin with), lightly tuned (again not bad) and switched the saddle to a new cannondale... sold in a few hours for $160
