The data processing part of this is going to be a bear - as will be the imaging, and communications -

This could get us a pretty good grip on the asteroid impact hazard - this needs to be done, and the US (we) could pay for this entirely, if we have to

What would be the total cost numbers, including medium launcher, probe propulsion, bandwidth, DP, and personnel? Under $1B ? Under $2B? $12 to $24 per famliy, and the asteroid impact hazard is dealt with?

The real problem is going to be dealing with the comet impact problem, including the carbonaceous chondrites.

Some Oort cloud probe, which would also allow for the study of gravity (clarifying the Pioneer anamolies), might be real useful as well -

The current key goal, which should be the main science focus for all of our existing planetary probes, is to get some new and accurate crater counts for all bodies in our solar system. With heavy duty analysis of all of these counts all together, this might finally yield some reliable estimates as to the total impact hazard.

Second and third in priority might come sun-planet interactions, and tectonic studies. Anything else?