There aren't any hard & fast rules - price is very dependent on vendors and whatever unique relationships they have with certain companies. For instance, you can get the Naniwa 2k green aoto in America for almost half the price it sells for in Japan. Also, keep in mind most brick & mortar knife stores in Japan stock a very limited selection of stones, mostly Shapton and Naniwa. To get most of the stuff we talk about on the forum, your friend would need to order online (Rakuten has some English support, and will allow him to pay COD so he won't need a bank account).
Sigma Power is one of the few bargain stone brands you'll be able to find in Japan, because for whatever reason, they don't have a dedicated international vendor. I'd look for the Sigma Power 1k soft version, which is supposed to feel very similar to the Rika, and much faster than a typical 1k. Also, depending on your lineup, you might want to look for the Suehiro Rika 3k and 6k, which come from the same series as the beloved 5k and aren't cheap in the states.
Similarly, Aritsugu (Tokyo) is the only bargain knife brand you won't be able to find (easily) online... Almost everything else (aside from the assortment of small undiscovered one-man smith operations) can be bought online in America, often for less than what you'll pay in Japan.