Welcome to WebmasterWorld. Did you read paynt's welcome post [webmasterworld.com]? Nice way to get started here.
I found subscribing to many business directories very profitable for B2B sites. Some company I promote spend 5 000 $ a year to maintain presence on several business directories. PR of such ressources is a factor but the main one is direct leads. This company recieves almost 40 % of all traffic directly from business directories. It was even better (almost 55 %) last year when we first appeared on such listings.
Most business directories let a company make online bids and proposals. Sales reps from this company where trained to use such ressources. (It also gives you more links by posting...)
I find 100/year to be the average fee. If you decide to hunt for such directories, I defenitely recommend you to use big directories such as Yahoo! and ODP. You will still find many with no inclusion fee at all. Start registering to these and look at your stats for a period of 3 months after. This will probably give you enough datas to base a decision on numbers.
Those kind of trade directories and B2B directories can be found in every corner and the weirdest sub categories imaginable.
I had success using DMOZ and Y! search boxes for such queries as "B2B directory" "trade directory", "vortal" and the likes.
Of course there is a lot of spam and scam out there. Using Yahoo and DMOZ is really a good start over using search engines.
When I started building my list of B2B directories and vortals, I visited them one by one taking notes of homepage URL and PR, submission page URL and cost of submission. If you put all these entries in some spreadsheet, you can have a good picture of what you can do for the budget.