When speaking about music that has been remixed from one genre to another, it refers to the amount of potential the new remix has in pulling fans from the original genre base.
A Britney Spears track (primarily pop) is remixed from it's original genre to dubstep. The crossover value of the remix relates to how many people would like or be interested in the dubstep version because they know the original track.
DJs can sometimes get people interested in new genres by playing remixes of popular tracks they know, allowing them to relate to the music and the lyrics since they are familiar to the listener. The ability a remix has to pull in listeners from its original genre can be referred to as the 'crossover value' of the track