The cost varies according to the size, purpose, and complexity of your website, whether or not your site leverages a lot of video or audio content (which would need to be reviewed and brought into compliance), what the firm you’ve hired to handle your compliancy audit and rework is charging, and many other factors. 

The cost of bringing your website into ADA compliance will be much less than the cost of a lawsuit if you’re found non-compliant. The consequences can include fines, monetary damages, court fees, attorney’s fees, and settlements; not to mention the hit your company will take to its reputation, which may negatively affect your customer base and sales. Nobody wants to be perceived as being discriminatory against the disabled!