Learn English – Word for a person who increases the argument

There are quite a few words that can be insulting about someone who doesn't like to spend money.

A miser is

a mean grasping person; especially : one who is extremely stingy with money

"stingy" is also kind of insulting, as it describes someone who is "not generous",

not liking or wanting to give or spend money

A scrooge is someone who is a miser and stingy:

her father is a real scrooge and refuses to pay her way through college, even though he can easily afford it

This word comes from the Charles Dickens story A Christmas Carol - Ebenezer Scrooge is a rich but very mean man who makes his employees work long hours for little pay.

Here's a picture of someone in the role of Mr Scrooge:

A "tightwad" and a "cheapskate" are also people who don't like to spend money. These words are a bit less negative, though: the person may not like to share or help others, but they are not as mean as a miser or a scrooge.

"cheap" is another good word. It is also used for things of poor quality, so it suggests a person of low morals or class who is also stingy.

Gullible is good IMO.

From http://www.learnersdictionary.com/definition/gullible: easily fooled or cheated, especially : quick to believe something that is not true. I'm not gullible enough to believe something that outrageous. They sell overpriced souvenirs to gullible tourists.

Credulous also fits, but from my experience/cultural-exposure, many people have told me they were (or someone else was) gullible, but I don't recall anyone describing someone as credulous. Strange.

Wikipedia has a comparison under gullibility:

• Gullibility is a failure of social intelligence in which a person is easily tricked or manipulated into an ill-advised course of action. It is closely related to credulity, which is the tendency to believe unlikely propositions that are unsupported by evidence.

There is also some comparison (and interesting discussion) of the terms in wikipedia's credulity though (IMO) the definitions given are not as precise as the various references suggest.

This ngram is interesting, showing that credulous has declined in the 20th century. This COCA report shows gullible is more common now.