「금새」 isn't a typo: it's a Korean word for the price of goods

When something happens within a short time, as in 「소문이 금세 퍼졌어요」 (the rumor spread in no time), 「난로가 금세 따뜻해졌어요」 (the heater warmed up right away) or 「라면이 금세 불었어요」 (the ramen turned soggy in no time), Korean spells it 「금세」. It is an adverb shortened from 「금시에」. 「금시」 is a Sino-Korean word (今時) meaning "this very moment," so there is an 「시」 hidden inside 「금세」.

So when you can't tell which one to use, try putting 「금시에」 in its place. 「소문이 금시에 퍼졌어요」 sounds natural, which means 「금세」 is the right one here.

「금새」 is a noun meaning the price of goods, or how high or low that price is. It is used in sentences like 「장마 끝에 과일 금새가 올랐어요」 (fruit prices went up after the rainy season) and 「이 집은 금새가 싸요」 (this shop's prices are low). It is the 「금」 of "setting a price" plus 「-새」, a suffix that adds a sense of shape, state or degree. That is the same kind of 「새」 as in 「모양새」 and 「쓰임새」, and it is explained as meaning the same as 「금」. It refers to a price set by the market or by haggling.

In daily life people think of 「값」 or 「가격」 first, so 「금새」 may feel unfamiliar, but it is a perfectly good word in the dictionary.

The 「새」 in 「어느새」, 「그새」 and 「요새」 is a shortened form of 「사이」 ("interval"), so it goes naturally with time. That makes people think of 「금」 as "now" and 「새」 as "interval" and want to write 「금새」, but 「금세」 in the sense of time wasn't built that way. The Korean spelling rules say to write by sound but in line with grammar, so even when the sounds are identical, the letters differ depending on which word it came from. Sentences like 「약 먹은 효과가 금새 나타났어요」 (the medicine's effect appeared quickly) are a common misspelling.

Here is a way to remember it. For "time passed quickly," use the 「세」 of 「금시에」. For anything about price, use the 「새」 attached to the 「금」 that means price.