Is “so” correctly used here? Must we use “too” instead?

Yes, so is correct.

'Too expensive' is natural, as is, "I thought it was so expensive that I started to leave.

Younger people do use 'so' when I would use 'too'.

'Too' is near almost impossible. for a poor people, Diamond too expensive but for a rich person it can be 'so expensive' but not 'too expensive'

So the sentence is grammatically correct, thought there are some slight differences in meaning?

Avidlearner'Too' is near almost impossible. f For a poor people person, a D diamond is too expensive, but for a rich person it can be 'so expensive' but not 'too expensive'

If the person, rich or poor, is going to walk out of the shop without buying it, then it's too expensive.

Avidlearner'Too' is near almost impossible. for a poor people, Diamond too expensive but for a rich person it can be 'so expensive' but not 'too expensive'

I'm afraid this isn't how it works.

It's "too" expensive = the cost is higher than I am able to pay or higher than I am willing to pay. I have $20 in my pocket, but I may say that an item that cost

I agree

You made my day, GG

wangqh2696122So the sentence is grammatically correct, thought there are some slight differences in meaning?

As your original sentence was written, it may be grammatical, but it is not natural.

Use "too."