yell pages brings in 3 x times the cost of the ad
£500 ad = £1500 new business
£2000 ad brings in £6000 new business
Thats how I have found it over the last 20 yrs anyway
OR if you get a job from yell pages ( new customer) thats worth £100, then about £33 is gone just on the cost of the ad
but,,,, its a new customer,,,so next time they book you or recommend you, you get 100% profit