Know-How Exchange : E-Marketing

Without knowing more, you have a couple of promotional options:

Natural Optimisation
Sponsored Links (AdWords etc)
Banner Advertising
Partnerships
Affiliates
EMail Marketing

Without further info about your particular business and objectives I can't help you. No-one can give you a budget without knowing what you are trying to achieve.

If you want more detailed help, if you need help understanding what to do, when, who etc contact me offline.

All the best

Andrew

AFFILIATES
One thing you could do is to provide visitors and local businesses with relevant newsletters/advice (i.e. how to optimise websites, moving business checklist etc), you could be clever and cross promote businesses on your listings that pay a premium rate for example.
Affiliates could be used to pull in visitors to sign up for the newsletter, then when you send it out you will get more traffic back, improved your traffic figures and help you promote your site more. Basically AFFILIATES= TRAFFIC, but I think you know that anyway.

BANNER ADVERTISING:
For your type of business you will need to place banner ads specifically on sites that send traffic to your competitors, Hitwise will be able to provide this data for you (it costs unfortunately), but I have used it with clients to 'hijack' competitors traffic and bring it to my clients site.

EMAIL MARKETING:
I guess if you develop the newsletter approach, then this is in effect what you are doing. You could also provide a better service to your paying businesses by getting some basic demographic and interest details from people who sign up.

Good Luck