The game has two design penalties that may catch some users off guard. These penalties are for over-designed and under-designed vehicles.
R&D Slider positions cause both issues, but they penalize you differently.
If too many of your sliders are to the right, I.E. better, then the designer will start to increase costs exponentially. The more your sliders are to the right, the more the exponential value increases. This increase is to simulate hyper-car designs. As the saying goes, you get what you pay for. It's very expensive to create the best vehicle ever.
This exponential increase becomes more pronounced later in the game you get.
On the flip side, if too many of your sliders are to the left, I.E. worst values, then your buyer rating will receive an extra penalty. This penalty prevents exploiting the game and to simulate minimum consumer desirability.
See the Buyer Rating Table page for more details about the possible decline in buyer rating points.
You’ll find designing vehicles, much like real life, is a compromise. You might want to produce the best vehicle ever made, but it would be way too expensive to sell. Likewise, you might want to create the cheapest vehicle ever, but the penalties to the buyer rating will kill sales of that design. The compromise is to balance making the perfect car with making the cheapest car.
Just good enough and cheap enough to sell well.