Or at an interval of say 6 months a piece? That way we will have low cost since the r&d will be basically 0 for all the extra models and we get 10x the data and larger time span of coverage. Plus we won't be putting all our eggs in the same basket.
The closest thing to what you are describing that NASA has done recently was sending 2 rovers to Mars at the same time (Spirit and Opportunity). It cost nearly $1B dollars in 2003 money to do it [1].
Nasa's entire budget for 2015 is $17.5B
In contrast, NASA only spent a further ~$130M keeping the rovers going for the last decade. R&D, as well as getting stuff into space, is incredibly expensive.
It would take months for each probe to get there, but once one arrives, you can transfer it from there to anywhere else in the universe far more easily than from the Earth's surface. And since those points are more stable than L1, L2, and L3, you can just leave the probe there for months or years until a good transfer opportunity comes up.
Here's a picture to get the flavor: http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA12021
So it's certainly _possible_ to build cheap spacecraft using mass production. You won't get anything specialised, so they're unlikely to work beyond the inner solar system, but you'll get cheap, simple and tested probes.
Whether this is _worthwhile_ is a very difficult question. Spacecraft cost money to run, and you're going to have to do very careful cost-benefit analysis as to what's the most efficient use of your money long-term. Plus, standard parts only work in standard environments; Jupiter's radiation belts would most likely kill Mangalyaan stone dead, even if had enough delta-V to get there, even if there was enough sunlight to run it.
But it's certainly worth considering designing a standardised long distance spacecraft bus, especially for missions to the outer solar system. Maybe someone's already done that...
(Mangalyaan is still in orbit, still collecting science data, still has years' worth of propellant, and AFAICT from the internets, has been an utterly textbook mission. They're sending another one at the next transfer window.)