There are apps out there that monitor the price of BTC, litecoin, DASH, Monero and compare it to all major currencies and even exchanges for bitcoin to altcoin. They have these listed in charts that go across a ton of different platforms depending on what app you have.
Wouldn't it be nice if one of them integrated the prices of different DNMs? You could choose to not subscribe to it in case you're worried about OPSEC but man it's a pain in the ass logging into every major marketplace just to see if the prices on your exchange are even remotely similar to what they're valued at on the marketplace.
You spending hundreds on drugs but waste time trying to save a few bucks checking rates on different markets? Alrighty then.