This is a great calculator that compares and contrasts how much it would cost to do TNRM versus having the cats caught and killed. Unfortunately these figures are for the US but it's a good idea to work out rough figures for your area if you ever need to talk to the authorities or the TC.
I know it's difficult to find out how much it costs when the cats are caught and killed because AVA has not been very forthcoming with their figures. Except for one newspaper article in the New Paper years ago, I've never really seen an estimation of the costs. However from calling pest control companies, you can get a rough estimate of how much it would cost (and calling a pest control company isn't cheap if you're calling privately - one would imagine that contract rates are cheaper). Of course if you know someone within the TC, or the pest control company, that is an even better source of data.
Bear in mind though, that these costs do not take into consideration how much is saved in terms of mediation and the fact that TNRM is not one-off. It does take a lot of time and effort on the part of caregivers. Factor in that most caregivers are even willing to pay for sterilisation, and it's not difficult to see that TNRM is the most economical way to go.