The contract might depreciate (it isn't clear what was contracted for), but ETH won't depreciate. It's the Currency, which is a different type of Asset. Tangible goods that "wear out over time" while in use is why they Depreciate = the value is lost a bit by nature of that use.

Currency is Held. It isn't "wearing out." It isn't "in use." It simply exists. All of your money in regular banks is not depreciating.

When you are paid by anything, including Chickens, you use FMV of that payment in exchange for what you were paid for. If I do $500 worth of work, but accept $200 value of ETH, then I got paid $200, not $500. And later, if I trade/convert that ETH for $5,000 US, that is the gain on the trade from $200 basis.

Is that what you are asking?