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This assumes you are relatively new on the web (so no grandfathering into paid directories). You are also typical in that you are fighting for 2 word keyphrase for the homepage (i.e. "translation services") but also have a bunch of 'product pages' that are good for a slew of 4-6 word keyphrases. ("website software translation services", "french english translations immediate").
I know this is asking 'how long is a piece of string' but I tried to make this question vague and therefore more useful for all the other newbies that come after me. Well at least for the next 3 months until everything is totally different :-)
Here's how you should use your $500.
1) Submit your site to DMOZ.
2) Join ZEAL and become an editor and submit an informational page of your site to a noncommercial category.
3) Pay Yahoo! $299.
4) Free submit your site to AltaVista, CrossWalk, AllTheWeb, Google, HotBot, Lycos (by signing up as a member first), MSN, and wait patiently, updating all the pages on your site regularly.
5) Go buy $200 worth of video games.
As far as I'm concerned, as long as it's not a FFA listing - it's good to have your URL on as many influential engines as possible (and to link back to that listing somewhere on your site).
You never know when the time will come that some major force in the industry backs a lesser one that you got a free listing on a year beforehand... and now they charge $299 just to get reviewed.
Google/Teoma anyone?
STEVE