Obtain a few quotes from different companies to compare costs, but make sure you use a registered migration agent. You will pay more for an experienced migration agent and reputable company.
Generally the costs are made up of:
Skills Assessment
English Language Test
State Sponsorship fees (if applicable)
Visa application charge (VAC)
Medicals
Criminal checks
Migration Agent service fees
Visa Bureau do a free online assessment too: https://assessments.visabureau.com/australia/assessment/skilled/default.aspx
Hope this helps