Internet advertisement is an lowest cost and most effective way to get customers first attention. At least for some products. If you open an internet shop without advertisement, you will probably get very few access to your shop even when your shop is listed in top 10 rank with proper key words. I know it from our 4 year experience. Lets compare to other marketing./sales methods.
*Contracting with big internet shop. There are two big ones, rakuten and yahoo shopping mall. Both require legal company entity in Japan. Off course there is monthly fee to pay. If your product is BtoC and can be stocked, not order made, it might be a good choice to contract.
*Hire sales person. Its a good way if you are selling a few costly products to limited companies not individuals. Things like expensive medical devices are good for this way.
Most of small to mid business in Japan use 5K to 10K USD for internet advertisement. Two major ways to advertise, google adwords and yahoo listing. Actually users are 50/50. But YAHOO is used by people with less computer addicts. Heavy users of computer use more google.
Interesting story. A friend of mine is a famous Android developer as well as a owner of a company who hosts a android teaching class. When he advertise for the same key words he says google costs more. yahoo is cheaper. But we sell key chains, coasters and straps and yahoo costa usually more. This means that Android class you get more inquiry from Google and small charms from YAHOO.