We have the Britax Decathlon, we bought it brand new, on ebay for $199 SHIPPED, brand new in the box, never opened. It was a great deal. It's basically a Marathon with extra padding that we no longer use. (We also have a Marathon.) I can tell you the seller's name if you'd like, since I'm not sure I can post publicly. You *can* get a new Britax for under $200.

I'd say get a Marathon/Decathlon before you get a Roundabout, they have lower weight limits, which means you'll need another seat sooner. Sure, they're less expensive, but it'll cost you more in the long run.
We also have a Cosco Alpha Omega, this one is in my mom's car. I also like this one because it should last her until she's out of a booster. She does sit a bit more comfortably in it, and complains less when she's in it!
I really don't have a bad thing to say about either seat. The Britax does seem sturdier, though. Both were recommended by consumer reports.
I'm sure someone can tell you what the weight limits are for rear-facing and forward-facing for both of these seats, I can't remember offhand since it's been awhile since we've purchased a car seat. Either way, you'll get longevity out of either.
Albeebaby often runs deals on Britax seats and they ship for free on orders over $100. If you don't see what you like right this second, and you have time before your DS outgrows his infant seat, keep checking back. Babies R Us is just plain old expensive, but Target sometimes has deals on Britaxes too.
http://www.albeebaby.com/britax-decathlon-convertible-car-seat-cappuccino-fabric.html This is $19 more than your limit, but they do ship for free. I wouldn't have posted it if it were more than $20 over.. but it wasn't, so I thought you might want to take a peek.
HTH!