The reality is that $8k/year isn't that much for a company. You can all share it, get quite a bit out even the smallest deployment and so on. However, compared to the work to slim down the minimum deployment for Spanner, this was honestly a reasonable outcome (that is, I certainly wouldn't have wanted to see the team delay a year to build a shared multitenancy version).
Additionally, Spanner does things for you that MySQL et al. don't. Having an automagic Regional (and eventually Global if you'd like) database without dealing with sharding is worth $8k/year even to me. So even if it could fit on $10/month of hardware, I don't begrudge them for charging a service fee, rather than saying "This is how much cores, RAM, disk and flash this eats".
Disclosure: I work on Google Cloud and obviously have a vested interest in you paying us :).
Basically yes, I'd like a shared Spanner. If $648/mo gets you 10k read/2k write, then for ~$9/mo you should be able to get roughly 140 qps read and 9 qps write, so long as cost scales roughly linearly. Assuming you can spread this over the month (1000qps here, 0qps there), this would be super appealing to a lot of smaller projects I think.
I think the way it is, it's exclusively a business tool but it has potential to be so much more. Imagine if it was possible for anyone to set up a distributed, fault tolerant, scalable Wordpress instance using Cloud Spanner, Cloud Functions and Cloud CDN any pay per uncached pageload? You could even set it up with a single click in the Cloud Console. Running your own scalable cloud service would be no more difficult than setting up a Tumblr blog.
But yes, we already have the per-op model for Cloud Datastore (and it's part of what makes it so attractive!) so you can understand that we're naturally inclined to do just what you're saying. Making Spanner multitenant in a secure, performant manner is real work. But it's what Datastore, Big Query and our other shared services do. I hope you can appreciate that getting "Dedicated Spanner" out the door was the right first step though.
And that's fair enough, I understand dedicated/shared are very different things, it's just that for me, right now, Spanner isn't viable. Change that and you'll be a whole lot more appealing!
So by pricing us out of the entry level, you will forgo having startups build their platforms on it and get hooked (while it's still at 1-node mysql scale).
When the sharding headache hits and they're scrambling to scale, will they switch to Spanner? Maybe. Probably. They'll want to, anyway. But there is a big market that will be missed.
But I think the issue is: making the commitment to use Spanner will take more than 2 weeks ($300 in credit) to determine if it's worth using.
Why not setup a "shared Spanner" with millions of pre-populated rows, and let free accounts have unlimited read-only access to it? That would let people fiddle with the technology before sweating bullets during the 2 week trial. (5 day trial if you run the recommended 3 node minimum)
Two node-weeks of Spanner credit, used wisely, might be enough to tell you whether Spanner fits your operations requirements, but it won't be enough to tell you whether you want to commit to creating an application architected around the paradigm it represents.