zami - Three or four years ago, the cost of converting a T2 to T2+ (it seems the nomenclature changes depending on whether the aircraft comes out of the factory as a P/T2i, or is retrofitted to make it a "+" designation) started at £130k - at which the members of the User Group, to which the announcement was made, all took sharp intakes of breath.
I'd be surprised if the figure is now much different, & doubt there would be much difference between a T or P conversion. RVDT correctly mentions a long list of modifications but, in effect, the real changes are the X-Y fittings (which may have been done already, but it's a comprehensive man-power job) and a different XMSN - if both are required, they're best accomplished at the same time, of course. The rest of it is mainly software changes and box change for VEMDs etc.
I've also heard from a colleague that, if you're looking at a Police role-equipped aircraft, you may have to consider some weight on the tail (approx 5kg, I believe) if you wish to maximise the AUM increase, from a foreward C of G point of view.