The £95m isn't the build cost. It was the total cost of building, maintaining and managing not just the stadium but the associated other sports facilities that were going to be provided (4G community pitches etc) over a 50 year period. Actual build cost for the stadium, incorporating NHS facilities and office complex for Sellafield was £26m. The leased space to NHS and Sellafield would have brought in roughly half of the £95m over the 50 years without taking into account other income the stadium would generate for itself (concerts, conferences, social events plus of course match day income). Plus the council would have recouped money from selling the Derwent Park footprint for redevelopment which is pretty sizeable. I read somewhere that after taking into account the rental income then even if the development never generated another penny then the cost to each council tax payer in the area would have been around 30p per week, hardly life changing sums.
The build cost for a straightforward 8,000 capacity stadium without any other facilities was £15m. This bigger plan was intended not just to provide a stadium for the semi-pro clubs but to provide other benefits to the community. Grassroots facilities, income into the town from the 500 or so Sellafield and NHS staff based there, improved healthcare facilities etc