Implementing a one to zero or one relationship
Scenario 1 could have multiple products linked to the same product_price. The second scenario, where the tables have the same PK (and one is a FK), is forced to being 1 to {0,1}.

Edit: Well now that you've changed Scenario1, my answer doesn't fit any more. Now, your two scenarios are identical, but your first table has two PKs. Scenario2 is fine and achieves what you want. If most products have prices, then you could also have them in a single table, which would look like the results of a Left Join, such that the products without prices just have NULL in those columns.