I'm also hunting down a Core II but I have a price in mind for just a loosey goosey unit(I happen to have a Core II pad already). You can look at sold listings on the auction sites, but I agree with Necro that "they are too damn much". I think the SSD3 really raised the value of them. If you looking for a box, then you're gonna be bidding against serious collectors with deep pockets, sadly.
Back in 2016 I got a Core 1 with a pad for 1300yen...but then after fees and shipping, I'm sure I ended up around spending $40-50. Best bet is to look for a lot with extra stuff (pads, games, etc) that you can sell off to recoup some $.
As for the op initial question of "what is the price"...it's up to what your willing to shell out. Personally I value loose cores+pad around $30 (before shipping) and I have no interest in any cardboard, so no thoughts there. Because I'm so cheap is why I haven't won a auction for a Core II yet