I get about 1 custy for every 250 leaflets but this can vary.
You need to try it for yourself because the leaflet design will make a difference, the area will make a difference, customers' expectations will make a difference, your price point will make a difference and so will your USP. It's not an exact science and will vary from area to area. Give it a try and see what works best for you. If you are thinking of expanding then you need to try all the different advertising media to see what works best in your area. Pick the one that gives the best return on investment and then go for it. You may find that leaflets don't work at all but canvassing is king. No one thing works for everyone.