If Mr.and Mrs. Hobnoby have you over for a four course gourmet dinner prepared by their live in chef, there is no expectation that you will invite them to a similar meal in your fourth floor walk up. Part of the reason God invented the cocktail party is so you can “reciprocate” many invitations in one big go that doesn’t cost you a lot per person. Unfortunately, a good hostess gift does not count as reciprocation- it is merely a (important) token of thanks.
Happily, you don’t even necessarily have to reciprocate with some kind of big event. Maybe you have helped someone move every year for the last five years, maybe you always do Friday movies and wine at your place, and maybe you are the person who arranges your weekly lunch date. All of that counts as reciprocation. As you can see, it’s really all about making sure you are both pulling your weight in the relationship and one person isn’t taking advantage of the other. Merely extending an invitation actually fulfills your obligation to reciprocate, and if you are refused, you needn’t do any more (however, this also might mean that the person doesn’t want to be friends with you!)
I should also note that it is important to also let people reciprocate your hospitality. Even if you intend to be generous, always wanting to host at your house doesn’t give others the chance to shine. Plus, you don’t want to give off the impression that other people’s hospitality isn’t good enough for you.
There are some exceptions to the need for reciprocity. You almost never need to really reciprocate with your parents and in-laws, the parent-child relationship is almost by necessity one where the sides are very uneven. However, occasionally treating your parents to something never goes amiss. I would also say that being treated by a friend’s parent is similar- when they take you and their child to dinner, you are really the guest of the child and should be reciprocating with them (hopefully you will also invite them along when your parents are visiting). Also, a boss employee relationship is one where if your boss takes you to lunch, you do not need to reciprocate (same as how you do not give gifts to your boss) because of the difference in power between the two of you.