I decided to "test" the drop rate of the new Murkmire plans from the envelopes that will become available through the master writs vendor next patch. Here's are the numbers I've got.
I created a new character so I could take the vouchers from it and used my main to buy and open the envelopes. Unlike the previous ones (SS and CWC), I have very few plans from Murkmire on my main, so from the 66 envelopes I only got 8 plans I already knew.
The 66 envelopes yielded 43 blue plans, 23 purple plans and I got 25 repeated plans. One of the purple plans dropped 5 times for me, and a few dropped 4 times.
Is it worth buying the envelopes?
For ME, yes it is. I consider myself a decent looter and I don't mind going over a zone time and time again to get what I want. Most of the zones will eventually "give up their secrets", and I'll manage to get the plans that way. Not Murkmire, though. I had better luck getting scuttleblooms there than I did furnishing plans.
Will those envelopes cause the furnishing plan market to collapse?
I don't think so. People were worried about this happening when SS and CWC envelopes were added, but I didn't see a huge change either. I think that most players either don't bother with the master writs vendor or don't meet the requirements to buy from her (and don't bother getting the achievement to be able to do so).
There might be a spike in purchase of such envelopes in the first few weeks due to it being a novelty, but I think it will taper off.
I'll try to repeat the experiment so I can have a bigger number to work with, but won't be able to do so when they copy the EU chars as I don't have a character that can buy them there.