It's unethical to charge someone one price for a recurring service and then more than triple the price, in an opt-out fashion.
If you raise prices, you need to do your due diligence to make sure your customers are aware they're paying more. This kind of a price increase should really be opt-in rather than opt-out.
I totally agree with you. The same thing happened to me with another registrar/hosting company (ipage). Hosting for a year went from around $50 to over $100 with auto-renew. I did receive an email notifying me of the upcoming renewal but no price was included in the email.
When I confronted them after the fact they told me that the information was on their home page where it says something to the effect of "Hosting only $50/year" and then right underneath in small print "(save 50%)".
Maybe I should have assumed that saving 50% now meant I would be charged twice as much the next time but I guess I'm not that bright.
Also, when asked for an explicit price list in a chat session, they gave me a link to a page buried in the FAQ to which, they acknowledged, you couldn't get directly in any easy way.
Gandi has been nothing but ethical in my dealings with them.
You don't have to deal with the abuses of the godaddys of the world, you just have to be willing to pay a bit more to support companies that aren't out to fuck you over.
If you raise prices, you need to do your due diligence to make sure your customers are aware they're paying more. This kind of a price increase should really be opt-in rather than opt-out.