It really doesn't matter what the breeder sells them for. It's the quality/conformation etc. which tells the story. What does the pedigree tell you? Have you seen the sire and dam? What are their temperaments like?
If everything looks good, then maybe it's a good buy, according to what your plans are for the filly. If you plan to show, then she will need to be registered (make sure sire and dam are registered). She will need to be top-knotch in the conformation dept. Do the breeders show? Are they well known in the breed? Have they produced top horses in the past?
The money you will be willing to pay, depends on all of the above.