In terms of value of discoveries there is a thread on the forums somewhere (I'll try to link to it if I find it) that give you a rough average of discovery values for stars and planets.
I just handed in quite a bit exploration data, some discoveries were in the thousdands, most in the hundreds of thousands and quite a few in the millions. One system on this trip yielded over 20m, can't remember it though but it must have been good as I had mapped quite a few planets.
The terraformable planets yield the most.
Found the link:
https://forums.frontier.co.uk/threads/exploration-value-formulae.232000/
My record at the moment is 23m for one system. In my journey around the galaxy I amassed 1.7b, however these days the amount you can earn for a given number of systems is much greater than it used to be, I think the amount for getting elite explorer needs to be adjusted.
Previously we got for money for honking, then money for DSS'ing. Now we get the same money for honking, plus the money for discovering the star, we also get money for FSS'ing, and money for mapping, and if you map the entire system you get a bonus for each body in the system, so it doesn't take much to get up into the tens of millions, in fact it doesn't take much to get to hundreds of millions. So even for a short trip tens of millions is easy money.
And of course if we die we lose everything, the money, the discovery tags, the mapping tags, everything, isn't life fun.