I think one funding model could be one that is linked to government spending, but then the reverse is also true. As there's a reduction in spending, then you would drive down the size of the office, which think would not necessarily be in line with the intended goal of ensuring good accountability.
I do think that you're absolutely right that government spending needs to be considered. As there is more spending, there is more work. We do do the financial audit work, and for the financial audit work, if the pool is larger, then it takes a little longer to get that work done.
I would just caution linking it, because then you have ups and downs that are really unpredictable, and then that doesn't ensure a consistent delivery of reports to help support Parliament.