I am going to slap my investors hat on and do a "fast valuation", that means it might be way off but is based on what I can see.
I reckon I get about 20% of the calls from Thompson as I do Yellow pages even though there is a lot less competition. Way less actually. 7 listed in Thompson, 47 in Yellow Pages. So I guess that close on 13%.
20% of 13% is 2.5%.
Which leads me to believe that round here that the yellow pages is used 40 times more than Thompson, however it could just be my area and my luck.
But from an investment point of view, I would personally spend £2.50 on a Thompson ad for every £100 I would spend in Yellow pages. I certainly wouldn't move far from those sort of figures, maybe triple (£7.50) I could not justify at all spending more than £20 for every £100 in Yellow, and I fear the real figure should be somewhere between £2.50 and £20 per £100.
However, you do need to factor in other stuff, such as how much value would that other 20% of work mean to you in those areas. What you can do is play crafty, because you have a low quote from Thompson you can go to Yellow Pages and ask them if they can match it.
Dunno if other people find the same with Thompson, ie 20% of the calls of Yellow pages and only 13% of the listings.