Will Dungeon Reward Chests change when you do it a second time?

Not that it will be a different weapon or an onyx gemstone.

But will it get like less valuable?

no, it's random.

the drops are completely random.

If you want an example, look at murakai's reaver, which in a chest green in cathedral of flames, due to it's appearance (Stygian Reaver dyed black) Many people are over farming the dungeon and thus more of them hit the market, more on the market means a decrease in rarity and thus also bringing the price down.

I hope that answers you're question