You are off subject. You asked why didn't the Germans make a huge production run of the 1960-65 Jagdpanzer chassis and use them as APCs (MICVs) to lower the unit price. The answer is that the HS 30 design was ordered in the mid/late 50s and was being manufactured and issued about 5 years before the Jagdpanzer chassis vehicles would have shown up.
The HS 30 chassis had a rather bewildering number of variants, both prototypes ( 12 different?) and paper proposals. It was also a bit small to be really practical at some of them and even made a rather poor APC/MICV. As would any updated 1944 Jagdpanzer without major modifications which rather blows a good deal of the supposed savings.
The German army may have wanted a better vehicle than the 1944 Jagdpanzer in 1960 or 65 no matter how cheap it was. While the Hs 30 used a gasoline engine NATO was moving towards diesel (unfortunately with a side detour with multi-fuel) but it did have an automatic transmission with 8 forward speeds. this 14600kg vehicle had a speed of 58kph and a range of 270km, both well beyond the capabilities of the 1944 vehicle. The 1965 Jagdpanzer used a V-8 500hp diesel to drive it's 27500kg weight at 70kph and had a range of 400km on the road. It also had an automatic transmission. While automatic transmissions are more expensive to build and buy to begin with they really cut down on the number of vehicles sidelined with blown clutches and wind up with lower costs over 10-20 years in maintenance and with a higher percentage of vehicles available for duty at any given time.