Common telephone question - how much to mow my lawn?
Answer - well until we see it we can't give you a quote.
Question - why not?
Answer - cos it depends on things like size, length of grass, amount of edging, clean-up required etc.
Response from potential customer - well it's a standard size block and the grass is only a little unruly.
Of course when you get there the block is half an acre with grass so long you can just see the top of the kids' swingset sticking out the top. You take four steps into the garden, trip over the dog you can't see asleep on the ground and fall in the pond you didn't know was there.