As well as the other replies, another cost aspect is the ground network needed to track and communicate with the probes. I remember several years ago NASA was

As well as the other replies, another cost aspect is the ground network needed to track and communicate with the probes. I remember several years ago NASA was aiming to reduce the network, in line with all the rest of the cost-cutting going on in early 2000s. I suspect that pressure has reduced, because we don't hear about it as much locally as we did then (we have one of the ground tracking stations here in Australia, and I lived in its territory).