Actually, there is a maximum Bitcoin price! How do we obtain it? Well, you can see that would occur if Bitcoin become the currency of the world and replaced all fiat.
Take the global M2 (total money supply including certain deposits: less inclusive than M3 but a good measure to be going on with), which is currently about $60-70 trillion. Divide this by the market capitalisation of BTC at the moment (let's say around $7 billion, because that's what it was when I last did this calculation and it gives a nice figure, and we're dealing with log orders of magnitude here), and you get a ratio of around 10 000. Thus, the price of BTC can only increase 10 000x from its current figure, unless more money is invented (which it will, by the time Bitcoin approaches that), but for now at least the theoretical ceiling of Bitcoin is:
$639 * 10 000 = $6.39 million/coin
Always fun to do back of a post it note calculations; have a good day