"That girl's bag is that cheap."
Translation:그 여자아이의 가방은 그만큼 쌉니다.
(1) Where did you see "여자애에의"?
(2) "애" is the collapsed form of "아이", meaning child.
As for "에의", I think (not sure) it might be a more general "의".
• 의, possessive case marker = of
여자아이의 = 여자애의 = of the girl = the girl's
• 의, noun = relationship, link
에의 = associated with, related to etc. can be used to mean "of, for, from etc." depending on the word which follows.
여자아이에의 = 여자애에의 = related to /belonging to the girl