lol, don't overthink this. Forget COR for now.
A human head does not have a consistent density all the way thru, of course. Without getting complicated lemme just put it to you like this. You need three components. You need to imitate the top layer which is quite elastic but thin. You need a skull, which is much more dense, somewhat flexible and quite brittle compared to a bowling ball. Then you need something to imitate the inner area(brain and surrounding fluids and whatnot). You can do a sh1t ton of research or you can just use easy approximations. Know what I mean? If you want an exact replica, you need to do a ton of work. If you just want a close approx. it's much MUCH less work. The skull will be the hardest to approximate.
Paper mache with a wood skull(you need to use the right wood) with a strong balloon full of jello or something inside the wood. Pack it tight, but not to the point to where it comes out of your wood skull.
With a lil more thought I'm sure some folks can come up with great examples. This one is just off the top of my head. If something comes to me, I'll let you know.
And no, the expensive realistic ones will not stand the test of time. They are like a real head except it wont heal in between sessions. Once you damage it, it's all downhill from there. They are typically for one time use for acquiring data on head trauma.