There are cards that I play in multiple decks, especially staples.
For example,
Beast Within is going to show up in every green deck of mine.
Another type of cards that I share between decks is lands. I have 3 Golgari decks and 1 Abzan deck, and I just don't want to buy 4x
Overgrown Tomb, 4x
Undergrowth Stadium etc.
What I usually do with lands above ~5$ is that I buy one, keep it in one deck, and proxy it in the other decks. I could play sub-optimal lands, but having a bad manabase is the worst feeling ever so I prefer to proxy.
Regarding more expensive staples, I usually avoid proxying. For example I only own a single
Assassin's Trophy, and I would include it in every single Golgari deck I have if I had more copies. However, I prefer to play it in one of my stronger decks (in this case, Meren) and play worse cards in my other decks (e.g.
Putrefy).
This will mean that some of my decks will get the more expensive staples and be stronger as a results, while other decks will get the sub-optimal cards and be weaker. However, I'm perfectly fine with this, as this will just mean that I'll have an arsenal of decks with different power levels, which is ideal if you want to play with different people/playgroups.