I would venture a guess that a true, national event level concours restoration on a 914-6 is easily in the $40-60K range these days. Could be more.
Otto told me once what he spent restoring the 914-6 he won the PCA National Parade concours with many years ago. It was a staggering number...and he did almost all the work himself, and got almost all of the parts at cost/wholesale.
That assumes you are starting with a reasonably complete and correct car to begin with.
A full restoration to me means that every part, no matter how small (i.e. every nut and bolt included) is essentially indistinguishable from new. Or better.
Few of us have even seen 914's in that kind of condition. There was one at the PCA-SBR concours a couple years ago. It was the green -6 car that was written up in Excellence (or maybe Pano). Unbelievable. It still had the original inspection marks and paper stickers on the underside (trans etc). IIRC it won at least its group at the event, maybe overall, with an almost perfect score.
This is the only lame pic I got of the car that day:
