I found it acceptable for myself when the main purpose of the trip is shopping (like it often is when I go with my family). To the Makuhari location it's about half an hour on a limited express and another 10 minutes on a local bus. Same with the Kawasaki location (except you don't need to pay an extra limited express charge).

The reason I go to these lengths to go to Costco in Japan? If I buy something there and it breaks, I can take it back to my local Costco when I get back to the US and they'll refund it. I love their global return policy. I should note that yes, I've tried it- my local Costco in rural Montana would take back merchandise I bought at Costco Japan's Makuhari location.


But back to the original question, I'd personally bring an even 100k to leave a little wiggle room. Probably no more than that if you don't like to shop at electronics stores (or Costco- I feel a lot less worried about plunking down 60k yen on a laptop if I know I can take it back to my home Costco if something happens to it within 90 days) or buy fancy designer stuff.