I think that the most common way to value Bitcoin is like valuing other currency by another currency.
Seeing to it that we all know all the factors that causes or affects supply and demand equilibrium.
Also the time we hold bitcoins. Time is very important factor.
We can do a lot of things with bitcoin . We can buy all we want. The mere fact that it is hard to earn atleast one is a reason enough to value it. If I would have one , I will manage and keep it well. Valuing it is like valuing our own money. All we have to keep it is to manage it and give our full attention to earn more.