To be fair, the price isn't just slightly lower - it is nearly an order of magnitude lower. Sending 500k messages a month with Amazon is $50, and with SendGrid it's $400.

If you send that level of email a month (which is easy to do if you send a weekly newsletter to a few hundred thousand users) you're now talking about saving yourself a few grand a year with Amazon.

Given the "switch" can be as simple as changing some SMTP settings, this is a big deal. I don't think the people who jump ship are necessarily hyper-price-sensitive customers. Amazon is just turning this service into a commodity.