The site does not explain itself the way you are trying to.
If someone is in the UK illegally, then it’s not a given that they are "flying under the radar" - that group are likely staying in hotel accommodation, waiting for their hearings on asylum claims/legal status and so, yes, of course they are costing (the UK tax payer) a fortune in the meantime. That meantime might mean years.
Others that have dispersed "under the radar" will have their own reasons for doing that, which are illegal in themselves and not likely not to be good ones in any case. Assuming that this last group will probably include a criminal element, then, in time, the extra cost of accommodation at HM Prisons will likely just add more onto the tax bill.
Looks like it's you struggling, not me, I'll leave you to it