I am reliably informed that the turnover from the 2008 Biennial was £750,000.
If the intention is to save £240,000 (which has mysteriously doubled from the £120K originally quoted), then as the Biennial is every two years, the argument for the yearly saving of closing Broadfield is probably completely negated. Also: if you take away the museum, then the Biennial is deprecated, meaning less visitors, resulting in a lower turnover.
The implication is that the Biennial creates more than enough profit to cover the cost of running the museum and still saves enough for the councillors to fly around the world on jollies.