The short answer is that the only limit to Bitcoins price is the amount of currency in circulation globally.
Lets start with the mathematics.
There is currently $136,916,431,899 (137 Billion) invested into 16,695,212 (16.7 Million) BTC which equates to $8,200 per Bitcoin.
Near $140 billion seems like a massive amount, but lets take some other investment markets into consideration:
There is over $1.2 QUADRILLION1 invested in derivatives globally.
The average daily volume on the foreign exchange market is around 5 trillion USD!
Global stock market is valued over 69 trillion USD. 3
The cryptocurrency market cap is currently USD 246,216,128 (246 Billion). In comparison to the other markets this is a fractional amount and has massive room for growth.
Back to Bitcoin, no-one can predict the future but lets assume that 1 trillion gets invested in Bitcoin over the next year, we are also going to base this calculation on a fixed supply of 16.7 million BTC (this is a rough estimation how many BTC will be in circulation in 365 days.
Current Market Cap: 137 Billion
Additional Amount Invested: 1 Trillion
Supply: 16.7 Million
= USD 68,000 (to the nearest thousand).
Now lets assume 10 trillion gets invested into BTC (less than 1/10 of the derivatives market).
Current Market Cap: 137 Billion
Additional Amount Invested: 10 Trillion
Supply: 16.7 Million
= USD 680,000 (to the nearest 10 thousand).
In order for each Bitcoin to be 1 Million the amount invested into 16.7 million would need to be 16.7 Trillion USD
While defiantly possible, I believe that Bitcoin reaching USD 1M is very unlikely, I believe that at the peak of the bubble Bitcoin will be near USD 100,000.