$0.10 per 100k writes is just a way of them billing you. Sure it's a lot compared to your avarage HDD price, but the data is hosted on at least 3 datacenters at all times, running google's software stack & everything is being constantly monitored by Google's SRE team.

App Engine pricing might seem expensive if you try to do a simple table comparsion with alternatives, but when you get more deeply into it you'll find that a lot of stuff that is included in the service with GAE will cost you extra when you use the alternatives.