Just like with basses there are a lot of good amps on the market.
Here are some things to keep in mind when buying an amp:
What will it be used for..practice?..a band...both
What kinda music?...jazz...rock...anything and everything
Combo amp or head and cab(s) separate
AND of course price....
My advice is to try out with your OWN bass what's available. I personally feel that an amp is simply to amplify your bass and that lot's of extras, such as compression, chorus, whatever built in is not needed. Your tone comes from your hands and bass and shouldn't come from an amp. You just want an amp that gives you the "truest" sound of your hands and bass...just my opinion.

I would say buy as much power as you can afford. I say this because unlike guitar, a great bass tone is usually derived by having enough headroom (power) to run your sound clean at louder volumes. Guitar players want to overdrive their amps, you do not. This is especially something to consider if you want the amp for band situation where you need volume. Obviously a rock band will probably be louder than a jazz combo...again application...
I think the $500 range can buy you a nice combo amp but if you need a bigger rig I would suggest checking out the used market. I know a lot of players are downsizing their amps so you can find some great buys.
Good luck!