If you really want to try them out, phone them up and give them your requirements. They will give you figures about how many people search for plasterers each month. When you hear the price, tell them that it's too expensive. Then ask them for a free one month no obligation trial to test the waters. If they decline, then offer them £10. At the end of the day, they are a business: whatever little money they can make they will take it, even if it's a loss leader in the hope that you'll return and give them more money.
When I advertise in magazines, I never pay the going rate. If they say to me it's £250 for a quarter page advert, I say "No way, that's so expensive. There's no way I'll make a profit if you're charging that much". Then the salesman will chat to you about the benefits and during this time you say to him, "I'll offer you £50 to test the waters. If it works for me I'll pay the going rate next time". However, when he calls you back in the next issue and asks you how the advert performed, do not under any circumstances say that it was great. You make sure you say, "It was terrible, I only received one phone call. I'll give it another go but I can only offer you £50" :RpS_thumbsup: