What are the differences between the punishments of the Shifchah and the adulterer?
Rashi (in Shabbos, 72a) and Targum Yonasan: If she sinned be'Meizid, she receives Malkos, be'Shogeg, she is Patur - whereas he is Chayav a Korban Asham irrespective of whether he sinned be'Shogeg or be'Meizid.
Why do the Shifchah and the adulterer receive different punishments?
Seforno: Because, bearing in mind that it is the man's Kavod that is affected and not that of the Shifchah, we assume that it was she who talked him into sinning 1 and not vice-versa.
R. Bachye: It will suffice for him to bring a Korban, whereas the Shifchah who has no money with which to purchase a Korban, receives Malkos. 2
Seforno: Consequently, she deserves Malkos, whereas he, who is close to a Shogeg, needs only bring a Korban. The Seforno on Pasuk 19 adds that in fact, it is precisely because of the different punishments that they receive and because the adulterer brings a Korban even when he is Meizid - both of which are unique to Shifchah Charufah, that the Torah inserts Shifchah Charufah in the Parshah of Chukim. Oznayim la'Torah: Moreover, Shefachos are inherently adulterous (See Avos, 2:7).
This is like the opinion of R. Yishmael, who holds that she is a full Shifchah. According to R. Akiva, she is half-free, in which case, she can have money (PF).
