"The chip is a Globally Asynchronous Locally Synchronous (GALS) system with 18 ARM968 processor nodes"[1]

So, 18 cores per chip. 55,556 chips. £270 per chip.

Still pretty good, considering the £15M is paying for more than a pile of chips.

[1] http://apt.cs.manchester.ac.uk/projects/SpiNNaker/SpiNNchip/