Absolutely. This at least puts them in the ballpark - double Cloudflare's $20pm, and something a startup can deal with. I'd work to avoid the $99 price on principle, but I'll consider this price based on reliability and benefits.
For example, I'd quite like to be able to offer subdomains for each customer. I switched that off once I moved to CF, because they don't do wildcards and I'd have to use the CF API to create each subdomain. Similarly, CF can't pass on the original IP address as-is so GAE can't tell me the geo area/city and I have to use CF's country indicator header. Both (I assume) go away if I move to GAE SSL.
However, with CF you get other features and you can add extra domains for $5 each. I have a couple of apps on GAE and CF's saving will add up quickly.