? -> $25
Back in March 14th 2013 the price was increased to $25 per month, as although the older price (which I can't remember anymore, was it $23?) didn't leave any headroom for advertising. Old subscribers were grandfathered in.
Here's what I wrote back then in the newsletter:
"By the way, in case you noticed the price on the home page changed, don't worry. Your price remains the same as before. For new subscribers it was increased a bit as a test to see if this would allow me to do a bit more marketing (it's difficult to spend money buying ads if the product is too cheap). If you had friends who were thinking about just joining, tell them to use the coupon code "buddies" and they'll also get the old price. What else... oh yeah, there was a bug on the site that prevented subscriptions for almost a month. "Oh I'm sure this small software update won't affect anything ^_^". That's been fixed now. "
$25 -> $29
September 9th 2016 the price was again increased, this time to $29 per month. Again the then current subscribers still got the old price. This time the reason was that in 2015 you could get 125 JPY for 1 USD. As that made everything cheaper for us, we started spending more money on product, postage and packaging. Then in 2016 you could only get 100 JPY for 1 USD, so all of the extra spending we were doing now suddenly meant that things were barely profitable anymore and there was no room for any advertising spend at least. No marketing effort means no growth, which means less scale benefits over time. I couldn't have that happen, so that is why the price was corrected to $29 to reflect the current situation.