The simple answer is that it depends on how complex and how long the affidavit is.
Short affidavit's to confirm to the court you did or said something may only take 10 minutes to draft and will have only a page or so of 'standard' content.
A substantial affidavit explaining technical issues or dealing with a long and complicated story setting out key evidence for litigation, can take many many hours and run to dozens of pages. Given that lawyers have to charge for their time, the former may cost under $100 while the later could in some cases cost upwards of $10,000. I personally have prepared an affidavit that cost over $6,000 to prepare, which ran to 20 or so pages of detailed history, but which set out the basic testamonial evidence for the case and included many revisions due to the witness needing to clarify numerous points which he'd attempted to gloss over in the initial interview. Indeed, preparing affidavits is one of the major costs of litigation, so if that is the type of affidavit you are talking about, the claimed fee may not be unreasonable.
When assessing whether your lawyer's charges are reasonable you need to think about how much time goes into preparing such a document, and then consider it in light of the rates your lawyer has charged. Although it may just look like a rehashed version of what you have said, the lawyer when drafting such a document has to ensure that the affidavit both reflects the information provided by the client EXACTLY and complies with the rules of evidence, which are quite complex, otherwise aspects of the affidavit may be struck out and this may fatally flaw the client's case. So keep in mind that not only does your lawyer have to draft the document, he has to do it in a rather meticulous way to ensure it complies with the myriad rules of evidence.
If after considering the above, you still consider your lawyer's claimed fee unreasonable, you can challenge the costs claimed through the appropriate process. Each state differs and it is not clear which state is relevant to your work, but you can find out the process either from your lawyer or your state's Law Society (called the Law Institute in Victoria).
I trust the above assists.
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