The probe itself is a relatively insignificant cost. The launch costs and timing windows are the limiting factor.
The tyranny of the rocket equation is the fact that to carry more fuel to orbit, you need more fuel to get it into orbit, which itself needs more fuel.
That's why gravity assists are so essential for escaping the solar system on a budget.
The planetary alignment that enabled the Voyager launch windows only occurs once every 175 years.
We should definitely be sending out more probes, but we can't just do it whenever we want, at least not without a lot more expense.
The tyranny of the rocket equation is the fact that to carry more fuel to orbit, you need more fuel to get it into orbit, which itself needs more fuel. That's why gravity assists are so essential for escaping the solar system on a budget.
The planetary alignment that enabled the Voyager launch windows only occurs once every 175 years.
We should definitely be sending out more probes, but we can't just do it whenever we want, at least not without a lot more expense.