Demand is the single factor which constitutes the price per BTC.

It is true that various traits of our cryptocurrency such as transportability, divisibility, homogeneity, scarcity and others, are factors that stimulate demand, but demand is the father of price.
There are also other external factors that play with demand, however those can only be speculated as to how much they impact.

Once the limit is capped, if everyone is using BTC there will be extremely high demand, thus each should be worth a lot.
Perhaps, 1+ Billion people / 21M BTC == a generous amount.

In-case anyone is worried, divisibility along with decimal shifting, solve the problem of: people > BTC

By then, you would be better of valuing your BTC in its native form, rather than 'so if I convert to USD, pay fees, buy-online with CC, extra fees' and so on.