Modern underwear, while comfortable and convenient, gives entirely the wrong silhouette under an historical gown. In order to achieve the necessary flat fronted chest (and massive cleavage) corsetry is required, and a farthingale and bum roll are needed to hold out the heavy velvet skirts. A shift is also worn to provide a flash of white at the neck and wrists, as well as to keep the unwashable overgarments clean.
Corset, farthingale and bum roll can be made relatively cheaply if absolute authenticity is not required. A fully authentic set of underwear is likely to cost around £100: using cotton as a substitute for linen in the invisible areas is likely to bring that price down to about £60.