I would say Nulltax is more valuable comparing to Nultax, as most people would spell 0 as null..
But you see, this is the first issue - it is not really clear how to spell it when you hear advertisement in radio, so it does not really pass the radio test.
But the domain is nice, it can serve as brandable for some tax optimization site, or services, I guess..although naming it particularly "null tax" is pretty bold to my taste Because of that, I would say it can find end user, but it will not be really big (and rich) company.
I would say mid-high XXX for right end user, maybe low XXXX if there is some good luck.