My business is built entirely on leaflets. If I drop 225 I get a customer (not a call). Mine contain a price, so almost every call is a customer, as they have already got over that hurdle. So, for me to get 100 customers costs 22,500 leaflets.
Ref delivery, I was sick of lying delivery companies (to hear more drop me an email), so I placed an ad on Gumtree for self-employed leafletters at £6 an hour. I got hundreds of responses and chose the two who had the gumption to look up my phone number and call. They carry a tracker so I know (1) the hours they have been out and (2) the streets they did (you can actually see their tracks up and down garden paths on Google Earth so you know they aren't just going for a stroll). They drift about doing the hours that suit them but if I wanted more customers I'd just up the pressure a bit and get a handful going. I'm pretty stuffed full at the mo so I don't need them going at it full-tilt.
They deliver around 120 an hour, so 5p each. My leaflets are just under 3p, so 8p x 225 = £18.00 for a self-qualified customer. I think that's an absolute bargain. £18 each for your 100 customers will be £1,800. You simply can't avoid the mathematical truth in there, I'm afraid.
As for waiting for the phone to ring, I get about 25% within a week, 25% second week, 25% third week and then the rest over the next year or so. (The exact numbers are a handful of % different but near enough).
So, 20,000 leaflets will be whatever you pay plus £1,000 delivery and at my rate of response would get you 89 customers.
Hope that helps,
Vin