Actually this is a manifestation of emotions in your head... If you don't share your cost price, it doesn't mean the price won't drop further right? And when price drops, it just mean someone has just sold, and someone has just bought at a price lower than you. There will always be undercutting unless you bought yours at the lowest point.
Then when rally comes, prices soar and you are in the green, but if you don't share your cost price ppl won't have a feel on how much profit you are earning, or how good your returns are.
And if you only start sharing cost price when market is in your favour, it will be like an ostrich burying its own head into the soil whenever it encounters an unfavourable situation, thinking that it can't see anything and thus nothing bad will happen but all these while things are still going on around it.