Is it, though? Let's do a quick and very superficial analysis of the costs here. Say we want to send a probe the outer solar system, like the moons of Jupiter or the Saturn system.
There are two unique challenges that such probes are faced with:
• on Jupiter, the magnetic field of the planet requires hardened electronics and shielding
• in the Saturn system and beyond, nuclear power is the only option for powering your instruments
Neither of these points are in any way shape or form correlated with launch costs. In fact, in order to get RTGs you need to build, maintain and operate an entire specialised nuclear facility. Alternatively you could use small nuclear reactors, but that may or may not interfere with certain instruments.
The cost of hardening electronics and shielding is also not going away even if launch costs were non-existent. It also doesn't matter much whether you produce 10 or 1000 of such specialised processors - the cost will still be orders of magnitude higher than comparable commodity options.
It's not getting any better if we turn inwards instead and consider Venus or Mercury. Both again pose incredible and unique engineering challenges as do most scientifically interesting targets in the solar system.
How exactly would a cheaper launch cost solve the problem of a Venus sample-return mission? Is it the expensive launch that prevents us from sending an orbiter out to Pluto? How does more and cheaper payload help with getting a probe into the oceans of Europa, Ganymede, or Enceladus? I could go on, but I think you get what I mean.
edit: there's also the human factor that I conveniently left out - every mission requires a staff of people (both scientists and engineers) for the entire mission duration; that cost is also unrelated to launch costs