Hi, I have bought lists from many data suppliers for many different clients across a range of sectors, including the Exhibition sector, and as a rough guide you will pay between 20p and 35p per individual contact. 20p will get you a name, job title, address and maybe either a phone number or email. 35p will get name, title, address, phone and email.
Depending on what you want to sell, choose your company carefully. If the product you are selling is a substantial purchase, about £5k or more, then you should think about getting more information when you buy the data. For a it higher cost you can also get the company's yearly turnover in £s and/or the credit score (a basic Good, OK, Poor, Unknown type rating) which will help improve success and cut out time wasters.
If you want a few good companies try MarketingFile or Electric Marketing. Marketingfile is good option to start for your inquiry - they use Dun and Bradstreet lists (very good quality) and price is good too.