You can determine the value between them by measuring how much of each ot takes to buy a carton of eggs. You can price any good in any currency. Take what it would cost in any local currency and then price eit with Bitcoin. I know there's not many people pricing goods in BTC or ETH or Monero but it doesn't matter. Eggs can be priced in USD, EUR, ETH, and BTC. Volatility keeps merchants from pricing in currencies that are monetizing or demonitizing. You don't see eggs priced in Gold for instance. But you can take any merchants current price in whatever they use and determine what it would be in any other currency. Yes USD is the starting point for most merchants, but not always and there are hundreds or fiats you can compare. Again it doesn't matter what you start in. After you determine value of the eggs in one currency you can do it for all others. Then you can equally compare any currency. It doesn't have to involve the starting point to compare. You're valuing the eggs in dollars, not the dollars in eggs. Now you can value the eggs in BTC, ETH, XMR. Then you can equally compare XMR to BTC. USD not involved in the equation. Things are getting rapidly cheapers with Bitcoin. The denominator is the unit of account. Just change your denominator for any good to BTC from whatever you see it in at the present. Now look at the trend. It's going up forever.