No it is not possible to calculate the price when the 21 billion cap is reached. The price might be the same, it might go up or it might drop. Supply doesn't always determine the price. There might be limited supply, but if there is no or less demand (that is no or less people are willing to buy bitcoin) for some reason, the price will be less. For some reason if bitcoin fails, then the price will drop to almost zero. If there were people that could find out the price, they would all be a billionaires by now.
Similarly, there is no way to calculate the future price of bitcoin due to its volatility. Price is basically not a problem, it really depends on supply and demand but one thing you have to understand is that bitcoin is not a ponzi scheme that will easily become 0.
Halving has an important role in the positive trend of bitcoin, usually people will be very bullish when the halving will occur and it makes the price often go up. The OP doesn't have to worry about the price because basically it will always go up and down. Just understand how the market works, you will know that it is this volatility that makes holders take profits and suffer losses.