That was probably before the change last fall [1] to have an SLA for single-"node" instances. At $0.90/hr per "node" for a Regional setup in Iowa, that's just over $650/month. Note that I put "node" in quotes, because it's hard to remember that "num nodes" != num replicas [2].
We "require" (I don't know if this is a hard requirement, but let's assume it is) a node per 2 TB of storage [3], so the example for Quora of "tens of TB without counting replicas" would probably be 15 nodes at 30T and say 50 nodes at most (100TB).
Assuming 50T as a balance, you'll need at least 25 nodes and let's assume regional pricing (since the MySQL example isn't going to be multi-regional by comparison). That's 25 x $.9/hr x 730 hours/month (~$16500) plus 50T of storage at $.3/GB/month (another $15k). So just over $30k/month or $360k/year. As for qps, we quote the multi-regional variants at 7k read qps per node (so 175k in this example) and ~2k write (so 50k).
That's all in though, so you'd need to compare to the cost of running a fleet of MySQL boxes too (at a minimum you'll also need ~75-ish boxes, just like the 25x3 for Spanner), 150T of raw storage (assuming 1 master and 2 read replicas per shard, which is pretty common) and then Zone to Zone egress (I believe Quora is on EC2). I'm assuming they're using gp2 volumes at $.10/GB/month, so that's the same $15k/month of storage. Being cost-conservative and assuming r5a's and 3-yr RIs, you pay about $18/month per thread, so we can buy up to 833 threads of r5a for the remaining $15k/month. That's maybe close to feasible (~11 threads per box), but honestly you probably need 16+ or even 32 for traffic this high (depends on query caching).
tl;dr: I don't think Spanner's pricing is higher than the sharded mysql fleet in this example! I do think it's not available on AWS or on-premises though, so that's a fine disqualifier for Quora.
[1] https://cloud.google.com/spanner/docs/release-notes#Septembe...
[2] https://cloud.google.com/spanner/docs/instances#available-co...