You can use wiki to get a baseline idea of prices, most are fairly accurate. You can use Ahungry also. No auction tracker of past or present ever has truly been 100% accurate...
Essentially the only way you can have a grasp of items ingame worth is to use the tools above and to personally watch EC with your own eyes.
As most facets of the game, to "get good" at tunnelquesting it will take in-game experience and time to learn the prices which is what makes the market on p99 challenging and fun.
Personally I like the fact there is no absolute marketing price website that determines exactly what every item is "worth."
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