The big deciding factor there is what you are going to feed with it, if you are going to feed the same stuff you are now but running it through the mixer I don't see it paying until you get at least twice that many cattle.  Now if you are able to use the mixer to it's full ability to make a good ration out of cheaper feedstuffs and you are able to find cheaper feed sources I think you could make a mixer pay with the number of cattle you have.  The biggest downfall I see to a mixer for the number of head you are feeding is the labor and fuel used which would be about the same if you were feeding twice as many head but that will also depend on how you are feeding now.  One nice thing about the vertical auger TMR's is used machines are about 50% cheaper than they were 3 years ago.