as stated in the film mellisa did privates, basically an escort service. this is actually where the pornstars make most of their money.
who loves all people of all gender,skin color, nationality, sexuality equally?
crocodile

Jane had her porn income in addition to the found money. She was able to pay $1,200 a month in rent out of that and keep a car, a pet, and have random luxuries like expensive sunglasses. For most of the movie we don't know if she has a job or what she'd been doing for money, but eventually we find out.
And she didn't seem to do THAT much splurging with the 10K. She got her nails done, tipped a cab driver, lent her roommate $300, and played some bingo. That's under $1,000, so the money lasting is believable if you accept that as all she spent rather than examples of her spending. Though I will give you that it doesn't add up until later in the movie when the viewer has more facts.
The roommate got her car back by prostituting, I forget the term she used for it but it clearly meant private sex/shows. The boyfriend was a drug dealer and handled all the roommate's earnings, so that's how he affored the living room stuff.