Cost of the satellite itself depends on the payload and on the attitude-control requirements - you can get a feel for rough costs by looking at the parts costs from suppliers like [1], adding in highly-skilled labor costs, etc. Numbers I've heard for simpler, smaller RF-only sats can be about $50K, while fancier things like Planet's Dove earth-observation sats (custom-built telescopes, precise pointing, high-bandwidth ground links) can run in the hundreds of thousands of dollars. (This is from what they've said to the press; Planet is very cagey about revealing precise cost numbers.)
Once you've built them, it's about $100K-$500K to launch them to LEO, depending on size (e.g. Nanoracks [2] will launch a 1U via the ISS for $85K, while Spaceflight Inc. [3] launches a 3U for $300K, or a 6U for $550K, or a 12U (who even builds those??) for about $1M).
So let's say a moderate-cost example would be a 3U with off-the-shelf payload hardware. Construction cost ~$100K, launch cost $300K, so $400K per satellite. You want a constellation of 50, that puts you at $20M.
For a more general Fermi-esque estimate, I'd say $2M-$200M depending on size and complexity of your constellation. Compare this to the cost of a single Falcon 9 launch at ~$60M, or to the $3.5B total cost of the Iridium NEXT constellation that SpaceX just finished launching.
[1] https://www.isispace.nl/products/
[2] pricing buried in http://nanoracks.com/resources/faq/, description of the service is at http://nanoracks.com/products/iss-cubesat-deployment/
I’m also curious if anyone can explain everything which goes into something like this and the costs.
There are quite a few constellations that you can benchmark by: - Astrocast (IoT) [1] - Fleet (IoT) [2] - Iceye (Earth Obs) [3] - Planet (Earth Obs) [4] - Spire (Earth Obs, Navigation) [5] - Capella Space (Earth Obs) [6]
And many more ...