I would say that the Kahr PM-9 with CT laserguard is the top subcompact 9mm option in the world today.
Were in in the market, that is
definitely what i would buy.
But they're a heck of a lot more than $500.00
A Walther PPS is another top candidate, though a police trade in $550-600 HK P7 is just as small. The HK will be a good deal heavier though, due to it's all steel construction. Weight is not really a big deal when you have a quality holster though, so it's not that big a deal.
Walther PPS with 7rd mag compared to 8rd HK P7.
Here's a pic of my custom P7 in my hand, you can see just how compact they really are, even with a laser and HUD attached:
