Hmm, I suppose it's fallen a bit out of favor recently, I don't see it quite as much as I used to, but it's pretty much standard terminology in the semi industry showing up in everything from academic papers to data sheets (commonly you'll see that as D$ and I$ - data and instruction cache).

Perhaps why I've not seen it before is likely due to localization - I'm in the UK and of course the Pound has it's own symbol, thus the connection of $ -> Cash -> Cache may not be so readily made.