The colloquial phrase wouldn’t give you a fright if he/she was a ghost is used of a miserly person.

This phrase occurs, for example, in the following from Christmas time is here by golly, deck the hall with hunks of holly, by the Scottish National Party (SNP) politician Steve Cardownie, published in the Edinburgh Evening News (Edinburgh, City of Edinburgh Council, Scotland) of Wednesday 6^th December 2023:

This Friday will see the various political groups up at Edinburgh’s City Chambers drinking, dancing, singing and, in some cases, talking, the night away at their respective Christmas parties.
[…]
I’ve organised quite a few in my time and found that getting a financial contribution from fellow councillors was fine in theory but a tad more difficult to put into practice.
One SNP councillor in particular wouldn’t give you a fright if he was a ghost, so it was a herculean task to get him to part with any cash.
He wasn’t that well liked either and rumour had it that when he was a child he had an imaginary friend that refused to play with him!

—Cf. also:
– the noun skinflint;
– the phrase to squeeze a nickel until the Indian is riding the buffalo;
– the phrase bang went sixpence;
– the imperative phrase let the moths out of your purse;
– the phrase to have death adders in one’s pocket;
– the phrase hip-pocket nerve.

These are the first two occurrences of the phrase wouldn’t give you a fright if he/she was a ghost that I have found:

1-: From the Lake Grace-Newdegate Cultivator and Dumbleyung and Kukerin Producer (Lake Grace, Western Australia, Australia) of Monday 2^nd December 1929 [page 4, column 4]:

Mean.

Brown: “I know a man who is so mean he wouldn’t give a fright if he was a ghost.”
Jones: “That’s nothing. I know a man who is so mean he grew a wart on the back of his neck to save him from buying a shirt stud.”

2-: From Apt Similes, published in the Taunton Courier, Bristol and Exeter Journal and Western Advertiser (Taunton, Somerset, England) of Wednesday 7^th February 1934 [page 10, column 6]—the readers had been invited to send “the smartest and best similes […] which they could either invent or unearth from any source”:

“He squirmed like a salted snail.”—P. G. Wodehouse, in The Strand. ¹
[…]
An old gentleman told his servant that he was as useful as a mirror is to a blind man.
A woman, speaking scornfully of a man, said, “He hasn’t got as much nerve as an artificial tooth!”
As empty as the streets of Aberdeen on a flag-day.
Mean? He was so mean he wouldn’t give you a fright if he was a ghost.
And those cats made more noise than a crowd of skeletons dancing on a corrugated iron roof during a hailstorm.
As useful as a refrigerator on an iceberg.

¹ This is a misquotation from The Nodder, a short story by the English author Pelham Grenville Wodehouse (1881-1975), published in The Strand Magazine: An Illustrated Monthly (London, England) of January 1933 [page 8, column 2]:

What had happened was that Wilmot, suddenly sighting his employer, had been unable to restrain a quick shudder of agony. It seemed to him that somebody had been painting Mr. Schnellenhamer yellow. Even at the best of times, the President of the Perfecto-Zizzbaum, considered as an object for the eye, was not everybody’s money. Flickering at the rims and a dull orange in colour, as he appeared to be now, he had smitten Wilmot like a blow, causing him to wince like a salted snail.

The phrase wouldn’t give you a fright if he/she was a ghost also occurred in the following two texts:

1-: In the following letter, published in the Manchester Evening News (Manchester, Greater Manchester, England) of Monday 4^th January 1988 [page 8, column 4]:

Gives nothing
I SAW the letter from Mrs Heartrey complaining about Margaret Hilda ² being unconcerned about the old folk being cold over Christmas.
You might as well have saved your postage; Maggie would not give you a fright if she were a ghost and she would not give you a shock if she had a power station.
T Berry (Mrs), Coppull, Nr Chorley.

² This refers to the British Conservative stateswoman Margaret Hilda Thatcher (1925-2013), Prime Minister of the United Kingdom from 1979 to 1990.

2-: In “a fascinating letter from Templeglantine”, in Card post-mortems can be the spark to ignite fierce conflicts, by John B Keane, published in the Limerick Leader (Limerick, County Limerick, Ireland) of Thursday 11^th February 2016:

I met Jack Faulkner at the Listowel horse fair. We spoke about decent men.
“A decent man,” said a customer in the bar, “is a nickname for a fool.”
“Do you know your man?” I asked Jack. Jack nodded. We both knew him to be a man so mean that he wouldn’t give you a decent fright if he was a ghost. His father and mother had the same reputation.