The simple answer, there isn't one.
Without knowing the exact weights, I would say it would have to be less than 2-3 kilos as a whole unit. Otherwise owners will start jumping up and down about lost payload.
Cost wise, will never happen. Hard enough to get owners to spend money on 30 year old aircraft now to keep them serviceable(just read through some of the threads about at the moment for an indication about aircraft not being maintained), let alone a luxury item like a HUD.
Just my opinion, and this is for commercial ops. In private ops, depends on the individual.
Would it be useful??
For IFR, **** yeah.
I suppose it' the natural progression though. We are starting to see glass cockpits appear more and more in GA aircraft nowadays, so it's a very real possibility that HUD units will eventually arrive.