If you want to get a vague idea of how much the space might be worth, you could look at
http://www1.thomsonlocal.com/displays-estimator/ and see how much Thomson Local would charge a business to get a premium spot in that particular category.
For example if I look at "plumber" and "cambridge" the tool estimates 3,522 displays a year at 11p per display i.e. it's going to cost an advertiser 390.72 pounds a year for a premium spot in the "plumber" category for the "cambridge" region. And searching the actual directory shows me that 6 businesses were willing to upgrade at 400 pounds each for that area/category combination.
And that in turn tells you several things. It puts one potential value on traffic within that area and vertical, and it also suggests that if you can get more to much more traffic than that, it could be worth disproportionately more to the businesses since they'll find it difficult to get such targetted traffic elsewhere.
To illustrate that second point, if I choose "self catering accommodation" and "cambridge" I get 1,212 impressions per year at 33p each or a listing value of 399.96
However, my own Cambridge site had over 37,000 impressions in the self-catering category during the same one year period. So that would give a listing in that space a theoretical value of over 10,000 pounds based on Thompson's math. At the moment I'm not doing so, but it shouldn't be that hard to sell slots at say 500-1,000 pounds a year by illustrating the vast savings to be made vs a large well-known directory.
Another place to look for some idea of pricing is
http://rates.yelldirect.com/InternetRates.aspx where you can dig down and see what Yell would charge a business for a premium listing in a particular category and area. They sell 3 slots per category, and have a specific price for #1 #2 and #3 position packages.
For example, they are charging £4459 a year for the top slot in "plumbers" "cambridge" (what they define as Cambridge is actually a fairly wide area that takes in parts of Norfolk and Suffolk. Again, this can help put a theoretical value on ad space.