Looking for a cheap sub...

to match with Boston Acoustic A150's on a consumer grade receiver (auxilary system). According to Boston, the a150's have a flat frequency response down to 30Hz - if you can believe them - but of course they tried to sell me the Boston PV600 sub. Budget is $300, so that pretty much knocks out the Hsu (VTF-2), Mirage and Carver (Knight Shadow) family. Any recommendations?

I got an NHT Super Sub used for about $300 last year. I've been pretty pleased with it, but I don't know how well it would mate with your Boston Acoustics.
You could get one as a kit in that price range but it would be a a passive model in that price range so you would have to drive it speaker level. The kits are very easy to put together,pretty much just have to install the drivers into the cabinet. The are very good bang for buck. It might be hard to find a good sub that would add much to your speakers if they are flat to 30 hz. for only $300. I looked in the for sale subs on Agon and found a few subs $300 and below but none I have listened to so I can't help you there.
FOr $300 there really is nothing out there that will add any significant extension to your existing system...I would wait till your budget increases...for a little more...you could go with a used Carver Jr...which has bass down to the low 20s..or even better...a used Rel...deep, clean bass doesnt come cheap...
Nothing is worse than a really crappy sub. I would hold out and get the HSU VFT-2. It really is a nice subwoofer, and will lighten the load on the rest of your system.

Your speakers are not flat to 30 regardless of what the specs say. I know "full range" 10k speakers that are not flat to 30. This sub won't be flat to 30, just a whole lot closer, while letting the receiver and speakers do what they can above 80. In the end your room probably won't allow for these flat responses.

You should be able to find a VFT-2 for very close $300. Anything over $350 is a joke for a used VTF-2.

I second Phasecorrect. If you can find a used REL anywhere close to your budget, I would jump on it.
I would also hold out for a used Hsu. Unlikely to find a used REL for under $500.
I have 2 of these. 1 I bought new the other I picked up used for $400. These can be had easily for < $500. I lived with 1 for 1.5 years, adding the second has really made the difference from a really good HT to an unbelievable HT experience. I'm not sure about using it for music as mine are dedicated to HT. I would say this sub's best quality is it's tight/quick bass, not boomy at all ( sealed cabinet design ).

I don't have experience with REL but did have at 1 time, an energy sub, a psb subsonic 6, the M&K is in a whole other league.

good luck.
With your budget of $300, I can recommend a Dahlquist PDQ1200 or PDQ1500, both of which can be purchased on Ebay for approximately $120 for the 1200 and $200 for the 1500.
I own a REL Storm, and purchased the PDQ1200 for $104 shipped(could not pass it up for the price!), and IMO it does a credible job considering the price!
Check out the Rava for $399.
You are the perfect canidate for a DIY project.I have had great success in that price range.Try acoustic-visions web site.You will find that 300.00 will get you brand new for a good value.You may get bit by that "big sub bug" and buy or build a true reference.