For loaded touring frames it's not much, the tubing is already heavy for carrying loads. There's little difference in weight between LHT and Disc Trucker frameset. Soma Saga DC takes both disc and canti/v so no difference there. In general, there's no significant weight penalty for the frames, on lighter road bikes there's a ~300gm addition for steel disc forks vs standard caliper fork. There are carbon fiber disc forks in the 400gm range for weight-weenies with $$.