In case people have a hard time finding prices for this, my vendor gives ~57k GBP ($85k) as the lower end price for configurations with that SuperMicro cabinet, 64 cores (8x 8-core CPU's) and 2TB of RAM. Dropping to 1TB lets you pick from a lot of cheaper cabinets and so the price drops by quite a bit more than half for the basic configurations.
As others have said they don't actually have a lot of RAM dedicated to Redis. You can put 1.5TB of memory in many 'inexpensive' Dell servers (single machines, not clusters). So basically a cabinet of machines you could have over 30TB of memory available to you. Basically some of the design choices Twitter has made seem tailored to their (previous? Not sure if they have their own hardware now) choices to run on 'cloud' services in the past.
With good hardware and a bit of a budget you can easily scale to crazy numbers of processor cores and memory. That isn't to say the software side of the solution is going to be any easier to solve.
