I've never had a watch where I was looking for thin or understated so I guess I'll just give you some possible sources.
http://forums.watchuseek.com/showthread.php?t=247781
This contains a good list of deal of the day sites for new watches. You can go through those once a day if you're curious. Prices should be ok, but used watches can be cheaper and better deals, located here:
http://forums.watchuseek.com/forumdisplay.php?f=29
Ebay have a good deal of watches, easiest if you have a brand in mind because there are a lot of crap around there. You do take a risk if you buy a used watch because no matter how big the scratch they will list it as "slightly scratched". Don't get the buy it now ones either, their prices are just about always bad.
Amazon are pretty much your only source of reviews for cheaper watches. Their price can be used as a source of reference but it's better to go to http://www.pricetrace.com/
and see what their lowest prices have been.
Hope it helps. You should be able to get good looking watches < 150, for your proposed budget of 500 you have a HUGE spectrum to look at so think carefully. I suggest you set your mind on a movement; I prefer kinetic or solar, but I see you already have a solar. Casio watches that are solar wavecepters are solar and unbeatably accurate but they're thick too. Anyways, hf on your search.