The teacher is wong and you are correct because you are using the subjunctive form "were" along with the present tense form "could" to.mean if it were cheaper now then you could buy it now. Many Americans say "if it was" nowdays though which is technically incorrect but considered the norm. The teacher's sentence means that if it were cheaper (yesterday, a month ago, etc), then you could have bought it (yestersay, a month ago, etc). So yours is for now / the present and the teacher's is for yesterday / the past.

For the second sentence you should use the third conditional form. The 3rd conditional form: If + past perfect, would(could) have + past participle. If the car had been a little cheaper, I could've bought it.

If you still have the opportunity to buy the car then it is correct to say "I could buy it" or "I will buy it" or even "I would buy it". However if time has passed, and you no longer have opportunity to buy it then it would be more correct to say "I could've bought it". The issue is with the tense of the verb. "Could've" is the contraction of the words "could and have" along with the past tense form "bought" indicating that the time of opportunity or chance to "buy" is already past.

Well, when you want to talk about yesterday, a month ago, etc, you should start with "If the car had been a little cheaper" or "Had the car been a little cheaper", shouldn't you? And the sentence would be "Had the car been a little cheaper, I could've bought it"