How much will one bitcoin cost when the 21 millionth bitcoin (the last one) is mined? All I know is that the price is supposed to only get higher the fewer coins there are to be mined.
Nobody knows. It depends on what the demand is for Bitcoins by that time. If everything goes as planned, the last bitcoin will be mined about 125 years from now, and I don't think anyone has a good idea what the world's economy will look by then.
One extreme possibility is that Bitcoin becomes the world's one global currency. To get a very rough estimate of what one bitcoin might be worth in this situation, we could note that there are currently 1.39 trillion US dollars in circulation. If all this value was to be stored in 21 million bitcoins, then one bitcoin would need to be worth roughly US$66,190. If you want the Bitcoin to replace other currencies as well, this number might be a few times larger. Then you should increase it some more to take into account the amount of extra currency that will be needed for the economy of the year 2140 (which, if things continue as they have been, will likely be much larger than today's, even adjusted for inflation).
Another extreme is that sometime between now and then, people lose interest in Bitcoin as a currency, and by 2140 nobody is using it at all except a handful of hobbyists, much like the situation in the first few years of Bitcoin's existence. In that case, one bitcoin might be worth only a small fraction of a penny.
Or, maybe in the next 125 years, we will achieve a post-scarcity economy, and nobody will have any use for the concept of "money". Or maybe humanity will be extinct. Nobody knows.