No, you only need to have an idea of the ballpark figure for a seat at a table. Then you bump it up by 20% to 50% and apply a multiple related to your own circumstances and level of personal connection to the guest. Then you buy a gift worth around that or just give them money in an envelope. (Some couples make their preferences known in this regard, but generally speaking I would say that giving a well-chosen gift is nicer than just giving money.)
It is not quite the algorithmic process I have described, though. It is a matter of custom and manners and therefore people arrive at the appropriate behaviour mostly unconsciously.
I assure you that if you personally do not have any idea about this, your +1 does.