A set of 3 lugs can weigh between 80 and 130 grams + -. A investment cast seat lug will include a binder bolt housing that would need to be attached to a fillet brazed or TIG welding frame. I weighed a unfiled Henry James down tube lug at 33 grams and a filed one at 22 grams.

Sometimes on bicycle forums posters will say a TIG welding frame will be lighter than a lugged frame and that is not necessarily true. It is likely the TIG frame is heavier because tubing has to be reinforced by the weld. The easiest way to do that is with an outside butted seat tube and a much heavier wall head tube. Those weigh significantly more than tubes used on a lugged frame.

One place where it is better to use lugs is on heat treated tubing like old Reynolds 753 or Tange Prestige. Lower temperature silver brazed lugs reduce the loss of tensile strength in the heat effected zone compared to bronze brazing. Of course Fillet Pro could be used instead or 853 tubing.