The average business metrics improvement after a usability redesign is 83%.

Small business example:
$2000 subsidized over 1 year = $167/mo.
Break-even = $167 / (100% + 83%) = $167 / 1.83 = $91/mo

Tech startup example:
$5000 subsidized over 1 year = $417/mo
New revenue = $20,000/mo * (100% + 83%) = $36,600/mo
Net increase per month = $36,600 - $20,000 - $417 = $16,183

Conversions: This estimate assumes that a "conversion" on the website is directly associated with revenue.

Cost of redesign: The cost of executing the site redesign (time, money and/or other resources) should be added to the cost of the usability review. Fortunately, the biggest benefits in usability are easy to implement.