Grupo SENDA has direct service from Guadalajara to San Antonio on their TURIMEX International premium service for $1450 pesos. You will cross the border in your bus and clear Customs and then go off to San Antonio. I do not know if you will be eligible for any discounts, but this avoids the hassle of getting from a Nvo. Laredo bus station to the bridge, walking across and then walking again to the Greyhound station in downtown Laredo. I am not aware of shuttle service from any of the Nuevo Laredo bus stations to the bridge, and given the deference to taxis I would be surprised if there was such a service. Perhaps that is something new. Otherwise you are talking about a $7-10 taxi ride.