Every relationship needs reciprocity.
But reciprocity in a relationship isn’t about you give me this thing and I will give you this in return. That may be a part of it, but it’s not sufficient. It’s not about making sure that everyone contributes the same thing either.
People have way too many differences for such a mathematical approach to be interesting. When entering a relationship it’s about creating more than the sum of two.
If someone buys a car, he wouldn’t imagine paying for it with a car, with money people invented a tool easing such transactions. The deal there is, that both see the transaction as a win for themselves. They’ve created more than they could have done alone.
That’s the crucial element of finding a price. It’s one that both see as an opportunity for themselves. One person sells something because having the money is worth more to him than having the product. The other person buys the product because it is worth more to him than the money.
Reciprocity is built in. It’s how reciprocity is dealt with in the economy.
It doesn’t work just like that in other relationships, there you’ve got to figure out what replaces “product”, “price” and “money” in your transactions.
You can try to stick to the rules of the market, but whenever humans are involved you’ll quickly discover that something is missing.