Communication rates with probes in the outer solar systems are extremely low (Voyager is ~160 bps), because of the inverse square law and the extreme distances involved. Achieving these rates consumes much of the probes' power.
Although rarely discussed, intermediate relay satellites (repeaters) can substantially reduce this problem by cutting the distance for each leg of the transmission. Creating such a network for a single mission would not likely be worthwhile, especially since repeaters at different distances from the sun would have orbits that do not sync up, requiring a large network. However, if the repeaters were designed to last for many decades, the network could be used by many probes, greatly reducing their power requirements for communications.
Has this been seriously studied? How frequent would interplanetary missions need to be in order for this to be worthwhile?