To find out what people are actually paying for everything other than Riven mods, go to Warframe.Market and check the prices posted by people who are Currently Online.

To find out what people are actually paying for Rivens on average, look at the official weekly logs of trade prices.

Even with that, Riven trading is still mostly guesswork and desperately trying to find someone willing to pay what the riven is supposedly worth, so don't stress over Riven trading too much, as it's the easiest thing to get stressed about in this whole game.

By the way, I still need to be added to the list of PC Tenno. I'm TheNeoVid in game, part of clan VGO.