From the start of project in 1972 through the Neptune encounter, the Voyager program (2 spacecraft) cost $865 million. Adjusting for inflation and rounding up to the nearest billion since there has been ongoing activity, that comes to $5 billion a pair in today's dollars.
Voyager 1 will be officially cracking through the heliopause anytime now, at latest by 2015. It's our first inerstellar spacecraft. If we wanted to build and launch an army of small spacecraft, how much would it cost? Using these numbers as our back-of-the-envelope starting point, the theoretical upper limit with these numbers is to look at world GDP, which nominally is $70 trillion. We'll still need to eat, so let's only use half the world's economic output. $35 trillion is 14,000 spacecraft. (Excessive? Half a percent is still 140 a year.) We can build ion engines on the cheap once there's a plant in space; the cost remains getting them out of the gravity well we live in. Orbital drydock would fix some of that.