The current prices of the vehicles are even better now than the way they used to be in RS4. I've seen many people already owning several vehicles like the ones you want, there's gotta be something that you are doing wrong if you can't earn money.
Sultan price is really cheap compared to the way it used to be and no, you'll never get a cheap Infernus and even I wouldn't like to see it cheap as it's the faster 4 wheels vehicle in the game.
The jobs are no longer a problem. With the bonus cash implemented on Mechanics, Drivers and Medics, almost all the jobs are kinda equal in relation to the salaries obtained.
PS: Elegy is a sportcar and it costs $30,000