It should be relatively straightforward to evidence the fully loaded cost of running the relevant team (it seems as though in many cases it is outsourced to TIL, which I presume already has some charging mechanism for the TOCs that use it), and from that an approximate cost per case, with some variation for different types of cost, to reflect the relative complexity.
By fully loaded, I would include all those costs that a business would incur, staffing, premises, IT, the share of associated shared services such as Finance. I could easily imagine you'd quickly get to £150 for even a relatively straightforward case, given the need to communicate in a manual way with those being investigated. (ie, it can't all be done by computers.)