Nobody knows. It's just supply and demmand. The more people that use it the bigger the demmand gets and the price should go up.
+1 There is no theoretical limit. There are some quite unrealistic ideas in the short term but the current market value indicates (to me ) that some large companies that use the valuable number as a payment processing system are being offset by adoption. Do not forget btc is many things and I use it as a store of value ie a commodity. However as an idea its unstoppable now in some form or other we just have to be a bit adept at countering the attempts of banks to keep their monopoly over the individual.