"Look at this car. This is less expensive."
Translation:이 자동차를 보십시오. 이것이 덜 비쌉니다.
I believe the second form is meant to be used as an object itself and the first is just an article that is attached to a noun
이 자동차: 이 describes the car, namely this car.
이것이: same origin as above, as in this thing (것), but overtime, it has evolved into this as a stand alone subject (as an object, it would be 이것을). I also guess that's the reason the space disappeared between 이 and 것. As usual, take what I say with a grain of salt.
The car is the object in the sentence hence the object particle 를 rather than the topic particle 는.