Comparable equipment has traditionally cost in the 10s of thousands. Only in the last two or three years has it been in the home hobbyist range.You don't need a SDR anyways.. unless you want to monitor multiple frequencies at once which is really the strong point of having a SDR in the first place. P25 isn't very wide. If you have any old police scanner with access to the discriminator output (or a "9600 baud" compatible Ham Radio that can reach the frequencies you want) then you can plug a sound card into that and go from there. I imagine it's even possible to define that as a source for the GNUradio program.