When available, Numrich lists the 770 bolt at $108 and some change.
Wait long enough, and one should pop up.
It is far from cost-effective to try using a Model 70 bolt.
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As for bolts coming up missing...
I've seen it before. Usually, it is simply a case of faulty memory, or the owner passing away, after the bolt was removed for "safety" or security reasons.
A few examples:
My grandfather, long ago, got into many heated arguments about gun safety with his gun-hating wife (it had something to do with a dismembered deer and what she saw as gun-toting hunters getting liquored up around a kitchen table...
). Giving in to the angry woman, he disassembled his firearms and stored them in pieces. Eventually, he decided that he should just give them away; but couldn't find all of the parts. It took many years before he collected everything and offered the piles of parts to family members that knew what to do with them.
Had he passed away before locating everything, we still wouldn't have all of the parts to those firearms. As much as my grandmother hates guns, and even their parts, there's no way she'd let us go looking for the missing pieces before she died. (Which she hasn't. She's hanging on for her goal of 114!)
My brother's father-in-law owns an M16 Berthier carbine (8x50R Lebel). However... he stripped the rifle and hid most of the pieces eight to ten years ago. As of now, we have located the stock, barreled action, and magazine "box". Everything else is still missing. If he kicked the bucket tomorrow, it would probably be when the wife also passes, before the house could be sorted, organized, and cataloged, and the rest of the parts were (possibly) located.
I bought a rifle a few years back that was missing the bolt. Why? Because it was dirt cheap (literally $8, minus the bolt). Why was it missing the bolt? Because it had ended up in a gunsmith's hands, and he didn't think the rifle was worth its weight in scrap metal. So, he sold the bolt and used the rest of the rifle to keep the stove pipe of his wood-burning stove wedged in place in his shop.
Heck, I've been there myself, now that I think about it.
At one point in time during my military life, I had a room mate that I didn't trust. I was too poor to afford a safe, but had two firearms in my possession. I hid the handgun in plain sight (very well, it turned out -- the room mate went looking for it once). And I removed the bolt from the rifle, in order to render it inoperable. The problem? When I needed the rifle for hunting season, it took me two weeks to find the bolt.