Yeah, that's what I thought. Here you go, both of these straight out of my Wish List on ZipZoomfly:
Western Digital WDH1CS5000N - My Book Home Edition 500GB USB/eSATA/IEEE 1394a 7200RPM: $125.99
750GB: (For only $10 over your listed budget)
Western Digital WDH1CS7500N - My Book Home Edition 750GB USB/eSATA/IEEE 1394a 7200RPM: $159.99
Anyway, I personally like those for externals. Some might disagree. But they have all the interfaces, including eSATA. I don't see where you could go wrong getting one of those. I'm personally thinking about holding out until the 1TB version gets a little closer to the $150 range, if that day ever comes. One can dream, right?
I know this is not a complete list. There are plenty of other options. Such as purchasing an internal HDD and getting an enclosure for it. That might be even cheaper. You can pick up a 500GB Seagate 7200.11 for $79 at TigerDirect right now, and I think the enclosures will run you about $35. Just something else to consider, because then the drive doubles as internal if you ever have the need.
Hope that information was helpful! Good luck deciding!