It's currently estimated about 3 million people own some amount of Bitcoin.
That's about 0.3% of the population of Europe and North America.
If Bitcoin becomes mainstream, and say 50% of people in Europe and NA own Bitcoin, how much would it be worth?
Assuming supply remains constant (to keep things simple), the increase of demand should reflect a 1:1 relationship, percentage wise to price. If demand goes up 10%, price should go up 10%.
What we can't assume is that each new person buying bitcoin buys the same amount. That starts to complicate the projections because it requires us to make assumptions based on more hunch and less fact.