Well, you would be able to find a lot in the $300 range.
As far as small tube amps go, you'd still be looking on the used side of things, but that would open you up to almost any middle-market 30-watt 112 (peavey, crate, etc.). You'd still have to find a really good deal, but it would be close.
As far as brand new, here's a couple.
18W Crate V-series
http://guitars.musiciansfriend.com/product/Crate-V-Series-V18112-18W-1x12-Tube-Guitar-Combo-Amp?sku=487051The Spider that jlynnb1 mentioned
http://guitars.musiciansfriend.com/product/Line-6-Spider-III-30-Guitar-Combo-Amplifier?sku=482275Peavey Bandit 112
http://guitars.musiciansfriend.com/product/Peavey-Bandit-112-Guitar-Amplifier-with-TransTube-Technology?sku=481332Those are just for reference. Basically, the $150 limit kicks you almost completely out of tube territory, but if that's something you don't care about anyhow, it's no big deal. On the solid-state side of things, you can't get into a decent wattage and a 12" speaker for $150.
I just look back at my purchases, and I spent so much money over time buying gear that I grew out of, or that wouldn't fit my tastes for the long haul, so you should try to think a little bit long term when making larger purchases.
I just think that if you are a lifer on the guitar, and you buy a 15watt solid state 8" practice amp, you'll be tossing it in the dumpster within the next year.
So figure out what you want (a ton of effects? Then get the Line 6. Touch responsiveness, and smooth saturation? Go the tube route. Don't need tubes, but not into effects? Then grab something like that Bandit 112).
Keep in mind I'm not trying to steer you in any direction, just giving you the spread. Go to a guitar store and play everything you can afford, and pick off of that--oh, and either take your own guitar with you, or grab one that is similar to yours. That way you know what it will sound like when you get home.
Hit back here and let us know what happens!