1) Let me see if I unterstood it right. I buy the pc and it costs me $1700. Then they refund me $1200. I should count the $1700 within my yearly expenses, and then consider the $1200 as a profit for that year, meaning I should pay 20% of the $1200. So if those two were my only expenses and profits for the year I should be paying $240 (20% of 1200) for khums, right?

2) I've been reading the rules about khums and it says that everything we spend is counted as our yearly expenses, except in the case we spend money beyond our status. What is the criteria to consider our expenses are beyond our status?

3) If I buy the pc, I would be spending an amount equivalent to 5-6 months of my salary, which may be considered beyond my status. But I would buy it only because they will refund me a part of it, and at the end I would spend only 1-2 months of salary (after they give me the $1200). So, is this purchase considered beyond my status?

4) Is the money spent on entertainment or non necessary things for living liable for khums? For example, if we buy a TV, go to the cinema, or a woman spends money on beautifying herself (in general, things which are definitely not essential for living), should this money be counted as yearly expenses or not?

I hope you could answer my 4 questions please, I've never paid khums before and I have many doubts about it.

1- yes absolute right , pay khums from 1200.
2- means any expense which is over the average of the expenditure of your income group.
3-if it is necessary and not any intention to avoid the khums then yes it is consider as expense and khums will not applied
4- Only make sure this entertainment is not haram if it is then refrain from it
Expense for beautifying is halal but remember that be moderate in it.