Got it - It's Neil Gaiman's We Can Get Them For You Wholesale , written in 1989. It's in several anthologies including Smoke and Mirrors , which is where I first read it. It's also been adapted for short films and graphic media.
From early in the story:
But the love of a woman can do strange things to a man, even one so
colourless as Peter Pinter, and the discovery that Miss Gwendolyn
Thorpe, twenty-three years of age, of 9, Oaktree Terrace, Purley, was
messing about (as the vulgar would put it) with a smooth young
gentleman from the accounting department-after, mark you, she had
consented to wear an engagement ring, composed of real ruby chips,
nine-carat gold, and something that might well have been a diamond
(£37.50) that it had taken Peter almost an entire lunch hour to
choose-can do very strange things to a man indeed.
Around the middle:
“Of course, but I doubt that sir would be interested
“No, no, I am. Tell me about it.”
“Very well, sir.
Bulk rate, four hundred and fifty pounds, would be for a large job.
Peter wondered if he had heard correctly. “Ten people?
But that’s only forty-five pounds each.”
“Yes, sir. It’s the large
order that makes it profitable.”
“I see,” said Peter, and “Hmm,”
said Peter, and “Could you be here the same time tomorrow night?”
And near the end:
“Well, I’m sorry it’s so late, only I was wondering… How much would it
cost to kill everybody? Everybody in the world?”
“Yes. How much? I mean, for an order like that, you’d
have to have some kind of a big discount. How much would it be? For
“Nothing at all, Mr Pinter.”
“You mean you wouldn’t
“I mean we’d do it for nothing, Mr Pinter. We only have to
be asked, you see. We always have to be asked.”
Peter was puzzled.
“But-when would you start?”
“Start? Right away. Now. We’ve been ready
for a long time. But we had to be asked, Mr Pinter. Good night. It has
been a pleasure doing business with you.”