It is pretty much a no no around here to give an actual gift at a wedding instead of a check or cash. I don't think I have ever seen anyone with anything other than an envelope and money at a reception or heard of anyone giving anything other than that for a wedding. Registries are for the engagement party and/or bridal shower around here.
As for covering your plate you can tell by the kind of place it is about how much it costs per plate and no reception hall is going to be under 75.00 a head in this area, and that is a cheap one!
I also had friends that would bring a blank check and depending on if it was an open bar or not that would decide how much they gave..if they had to pay for their own drinks the gift went down LOL.