Mid 6 figure DB bill, let's estimate $500k. Divided into 100 million rows (ignore the rest, because db provisioning is typically dominated by the needs of a few core tables). They get 200 rows per dollar.

Your table on a small VPS (which I concur is totally reasonable, am running something similar myself): Let's say your VPS costs $40/mo x 12 = $480/yr. Divide into 150 million. You get 312,500 rows per dollar.

I'd wager you server was faster under normal load too. But is it webscale? /s

There's waste, then there's "3 orders of magnitude" waste. The pain is self-inflicted. Unless you have actual requirements that warrant a complex distributed database, you should "just use postgres".

And just to calibrate everyone's expectations, I've seen a standard prod setup using open source postgres on AWS EC2s (1 primary, 2 replicas, 1 haproxy+pgbouncer box to load balance queries) that cost ~ $700k annually. This system was capable of handling 1.2 million rows inserted per second, while simultaneously serving thousands of read queries/s from hundreds of internal apps across the enterprise. The cost effectiveness in their case came out to ~ 20k rows per dollar, lower than your VPS since the replicas and connection pooling eat into the budget. But still: 2 orders of magnitude more cost effective than the hosted distributed hotness.