The word is new to me. It hits some 45,000 sites in Google. This number is not so large as a frequency of use of a single word. I believe this sort of slang will be soon out of use. An online slang dictionary says as below.
ch-ching. This word means excellent, fantastic. It is believed to come from the sound falling coins make when
That usage is pretty outdated. Lately it means a lot of money. I'm convinced that the sound is that of an old-style cash register pushing up the display numbers and opening.