how much your fiancee needs depends on how long she is planning to stay and what she needs to pay for

this gives you an idea ..
http://www.londonelegance.com/transp...tionLetter.pdf

just as , if not more important is convincing the embassy your fiancee will return before her visa expires.

also her being your fiancee can sometimes be a double edge sword, a reason for her not to return because she's in a relationship with you, and if she doesn't mention she's in a relationship with you, it could be used against her in future visa applications for withholding that information.