There are more options if all you need is WordPress. ZippyKid and page.ly both seem to be high quality and economical.
But the problem is that AWS pricing itself is not completely transparent. It's based on usage, which means that the bill at the end of the month can end up being an unpleasant surprise, as it has been for lots and lots of AWS users over the years. Who can know in advance what their usage figures will be?
RAM-based pricing means that the bill at the end of the month literally cannot be a surprise. We have four available plans, and monthly bills map directly and seamlessly onto those plans.
We run on AWS (among others) but WE pay AWS, not our customers, because we think that part of the mission of PaaS is to shield customers from the many disparate pricing models on different IaaS providers. Want to run on Rackspace via AppFog? Same price. Want to run on HP? Same price. Azure? Same price.
PaaS needs to abstract hardware away completely, even monetarily. Paying IaaS providers directly is not the way to do that.