Answer:
Product Placement can range from absolutely free to $50 million for one film. It just depends on the situation. If a Production Company, Director, or Writer needs to show a brand in a specific scene they may simple just want to use a brand they like themselves, which would cost nothing. (These instances are few and far between for most brands since many of the larger film and TV projects have brands lining up to pay to be in them.)
It also depends on the amount of exposure a project is expected to get. Brands can also expect to pay more depending on whether the product is handled by a star, whether the brand is verbally mentioned, and whether the product is featured "in focus" in the foreground or background. Generally speaking, if it's an unknown webisode on some unknown website, placements won't be worth much if anything at all to a brand. However, if it's a sequel to a film that earned $800 million like "Spiderman 4", a brand should expect to pay millions for good exposure.
The most expensive deals on record were for placement deals with James Bond films. Ford and Aston Martin reportedly paid $50 million to have their cars shown throughout the film. Bond films earn excellent money with placements because of the high tech theme and worldwide appeal of the films.
In many cases, a brand or ad agency pays a product placement firm a yearly retainer fee to read scripts, find fitting projects, arrange deals with producers, and deliver products to sets to be placed. This cost as well can depend on the brand and the desired projects. Cars, shoes, and beverages will be paying more than most due to the competitive advertising markets for their categories. The films that would likely cost the highest would be "Iron Man 2 or 3", the next James Bond film, "Spiderman 4 or 5", and films like this since they will earn huge box office grosses due to their worldwide releases, popularity, and film franchise track record.