For people doing multiple playthroughs:
When I played Mass Effect 2 the first time I mined every single planet and had over 400000 iridium, over 300000 platinum and palladium and over 100000 Element Zero, but I ran out credits and couldn't buy all of the upgrades. So in one of playthoughs I bought all the upgrades and didn't research anything until right before the Omega 4 Relay and came up with the approximate amount of each element you need to research every upgrade.
Element zero- 35000
If you have an import save from Mass Effect 1 and beat Mass Effect 2, you get up to 50000 of each element, so you won't even need to mine for element zero. So basically you can mine until you get these amounts then won't have to waste any more credits for mining probes. You will also get more elements from containers while doing missions.