you could probably get a new entry level Specialized for $300-350 on sale...which isnt great but waaaay better than anything at Walmart or target. But for $150 i second the recommendation of buying used.
Giant as well. Same company as Specialized but cheaper in price, not quality.
Stop by a locally owned and usually smaller bike shop in your area. They sometimes have used bikes for sale. They will have worked on the bike to get it up to snuff and will want your business in the future so they are less likely to screw you on the price. It will be more expensive than random bike on Craigslist but the tune up and basic maintenance is done already.
You won't get a half way decent used hard tail for $150.00.
Bump up to $200-$300 you might find a decent Kona Blast or Trek 3700, but it'll probably be junked out.
You need to get up to $300 $400 for a decent used CL find. GT/Trek/Giant/Gary Fisher/Kona - they're on there.
I picked up a 3 yo. Giant Yukon on CL for $350.00 It had an Azonic HB with cane creeks grips. Seller threw in some slicks for good measure. I lucked out. It's been a great bike.
No ****, overmaars. Nice find! Keep that dude's number in case he has another bike I can pick up. sheesh.
OP, be patient. Good deals are possible but sometimes you have to be patient. You also have to be consistent with your search. Check daily or a couple times a day if in Austin or a cycling town. Good deals get snapped up.
Check if any of the local dealers get bikes that are overproduces by the bike companies they sell. I was able many years ago to buy a trek 5200 which sold for $2500 and got it for $1700. I believe trek has several of these kind of bikes to sell every year in all range of bike types.