I believe the value of NMR is simply contingent upon its demand by those wishing to stake it for a sum of the prize pool. One would assume that the more money Numerai generates and, as a result, the greater the prize pool, the greater the demand for the NMR token. Nonetheless, I've read that they would rather the NMR token be in the hands of users. This might be (at least partially) the reason as to why they did not do an ICO.