I did this mostly for myself while debating whether I should pull for this event or not. Thought I should share the estimate for those who are debating, especially newish F2P players.

So with cheat cards from the Nightmare events, you'll definitely unlock rewards faster when the pop quiz goes live. However, does the cost of summoning from Nightmare events in terms of Devil Points actually surpass waiting for the Pop Quiz to return on Lonely Devil for you to do re-runs?

From some of the calculators fellow players have made available, DP from re-runs range anywhere from 350~700 for the later events, possibly going up to 800~900 with the raising criteria nowadays. For the sake of forecasting, let's round up and say completing LD re-runs (not just for the UR) might take you 1000 DP.

Meanwhile, summoning for cheat cards, with 1000 DP, is equivalent of about 50 pulls if you do sales. Plus the DVs you get from the process, you might get around 75 summons. Now for my last event, I did 100 pulls, but didn't manage to get one of the UR (Mammon whyyyyy). This actually set me back such that the Pop Quiz UR was out of reach.

Now for Spy, as much as I want the Lucifer UR of the event this time, I don't actually have a need of that UR in terms of advancing the main lesson right at this moment (I have 3 Gluttony URs that I just need to level up), nor do I need the ones from nightmare. So... looks like I'll be keeping my vouchers this time.

TL;DR Only summon from Event Nightmares if you WANT the event nightmare cards. If you're doing it for the Pop Quiz UR, it is very likely cheaper to wait 6 months for it to return on LD.