Linux + MariaDB (Fast MySQL).
Enterprise level throughput + scalability.
Cost == $0.
Well, cost included with whatever hosting you're using.
I run a private hosting company. Some of my machines are running 250,000+ uniques/hour + have been for months.
MariaDB can handle whatever load you throw at it...
Well... so long as your coders know how to write high speed code.
Most coders are clueless about this.
Tip: Pay for expensive developers up front + you'll save huge amounts of money avoiding rewrites.