Yellow pages ads are complex. Prices depend on a variety of things. Which book you advertise in (assuming there are multiple books in your area), how large of an ad you place, etc.
If you are going to advertise in the yellow pages you want to make sure and do your homework first. If you have multiple books in your area, you want to look at the top two and sit down with a sales person from each of them and go over the info they give you. Each of them are going to claim that they have the most readership typically. But you want to read between the lines. You want to have that info verified with an outside source (my favorite is the National Yellow Pages Monitor). Whoever REALLY has the most readers, you want to advertise with them. Although you should expect to pay probably twice as much for the ad.
Another giant factor is the size of your ad. Small ads get very few calls. And in my experience, are not worth the money you spend on them. If you are advertising in the "Lawn Care" section, you will want to be in the top 3 as far as size of your ad. Anything less and you'll get lackluster results. Look at the stats. The #1 ad in any given sections of the yellow pages gets like 50-60% of the all of the calls from that section. The #2 ad gets like 25%. The #3 ad gets 10-20%. The remaining goes to the lesser ads. And it aint much.
If you are advertising in the "Landscaping" or "Landscape Contractors section" (assuming you do that kind of work) then you don't necessarily need to be in the top 3. You probably won't be able to handle the calls. You only need to land a few decent sized landscape installations per month and you'll be plenty busy. So you can get away with a smaller ad. Anywhere in the top 10 or so ads will suffice and probably get you enough business to pay for the ad.
Last year we were #1 in the "Lawn Care' section. It worked pretty well but as Kent Lawns said above, you weed out a lot of them. And since we were only advertising for lawn care and maintenance, I think we got just barely enough business to make it worth it. Still, I wouldn't do it again. It wasn't quite the results I was hoping for.
This year we are paying $750 for the #8 position in the "Landscaping" section. I have only received a handfull of calls so far as the book just came out a month ago. But today I got a call for a full front and back install and I bet I'll get the job. So it appears to be working ok. We'll know for sure in about 2-3 months.
All and all, Yellow pages is not the best return for your bucks. Fliers, for the money, are by far the best return on your dollar. I could pay the same $750 for someone to lay a few thousand fliers per month and probably get more results than I am from my Yellow Pages ad. The problem is that I don't have someone do that and I get too busy in the spring and summer to worry about making copies of fliers, sending someone out to do it, etc. I like the fact that the yellow pages demands none of my time after it's placed.