We got a wedding invitation from someone we barely new and it was out of town we sent fifty dollars and got no thank you we attended a wedding and fancy sheva brachas we gave 360
Many brides are registered
You give according to your means. Maybe more for someone close and less for someone more distant. More if you attend the dinner, less if you don't. Some people send nothing if they don't attend, others give something. I send nothing if I barely know the people and am not going to the event, unless I feel sorry for them because they're poor and then I send something more as tzedaka than as a wedding obligation. If the couple is registered, you can send something from the registry, but be aware that many couples just go wild registering and end up returning much of the stuff, plus you pay for shipping and that's a waste IMHO. I'd rather give a check; I know they won't return that.
There is no obligation to send a gift for the wedding of someone you barely know and won't be attending. Such invitations are a shameless bid for a gift IMHO. Business acquaintances, that depends. It may be part of the "one hand washes the other" game: you send my dc a wedding gift and I send more business your way. If they sent you a gift it would be churlish not to reciprocate.