The question is too generic. 
The charge for BAS Preparation will depend on many factors.
What business structure, how much data, and if payroll is included.

The cost will depend on the following:
Is it a sole trader, company, or trust?
Different structures require different compliance parameters. 

What level of service do you want for BAS preparation?

Simple BAS preparation from $88 plus a month
Excel sheet or accounting software provided by you, the client
Bank Feeds coded by rules
Bank reconciliation
No payroll or superannuation

Mid Range BAS Preparation from $150 plus a month
Accounting software supplied with the smartphone app to capture receipts
invoices/expenses matched and/or bank feed rules
Bank reconciliation
Possibly 1 employee + super + STP
Quarterly BAS as part of the monthly package or charged separately

Higher Range Services - Usually requires a quote based on workload + payroll requirements
Accounting software supplied with the smartphone app to capture receipts
All invoices/expenses matched to the bank feeds
Bank reconciliation
Staff Payroll + Superannuation + STP
BAS completed as part of the package or charged separately

Other services that may be included in a fixed fee package, or may not
FBT installment 
Luxury car tax
Fuel tax credits
Taxable Payments Annual Report
PAYG Annual Reporting
End of Financial Year reconciliations and Reporting