Here’s a nice overview of some real-world examples of what it takes to make documents accessible and compliant with Section 508:
Child Abuse Prevention Resource Guide for 2013 is over 80 pages (see https://www.childwelfare.gov/pubs/guide2013/guide.pdf). From design of this document to PDF conversion, this report required about 20 hours of work to ensure compliance.
Child Maltreatment 2011 and 2012 reports (http://www.acf.hhs.gov/programs/cb/research-data-technology/statistics-research/child-maltreatment) are examples of ~250-page reports that required about 40–50 hours to fix Section 508 compliance issues and address any resulting formatting issues in the PDF.
Child Welfare Outcomes reports (http://www.acf.hhs.gov/programs/cb/research-data-technology/statistics-research/cwo) are over 400 pages, with many data tables, and may take well over 80 hours and $6–8K to make Section 508 compliant.
Overall remediation budget for about 200–225 PDFs of various sizes and complexity, most in the 2–20 page range with perhaps a quarter in the 21–200 page range, is about $250K. If most of the documents are at the high end of these page ranges, the cost (assuming a loaded rate of $80/hour) would be upwards of $300K.