in the UK and perhaps other European countries it is not a controlled substance or even prescription only, and can be bought from pharmacies (but not other shops). The staff there will ask you questions and buying loads at once will make them suspicious and they will refuse service, but there are many legitimate medical reasons for taking it. I expect it would not be more than about €5 for 20 or so tablets anywhere, but Europeans can currently get far better similar drugs with less overdose risk for the same price…
it doesn’t even seem to have been heavily restricted in the USA, any controls being voluntary decisions made by commercial companies. So the street cost would merely be the cost of it from some area where it can be bought legally from a drugstore, plus the cost of petrol (gasoline) or other transport costs to get the stuff to wherever you may be (as the USA is very big I can understand why it might be sold like a street drug).
