As a Vanguard i took armormech. Recently i was doing some Flux quests just thinking it was cheaper. Conductive flux sells so 10 creds, my scav had me pay 215 creds (i think) I got 15 flux three times doibg this, that is not cheaper.
The next lvl flux is 50 creds per, i did a few quests of them.. missions went for500ish creds, i got 6 to 8. I will recheck tonight when i get home but i think its just cheaper to buy in bulk than mission for the flux.
I'm not sure what this is in response to but my opinion is to only take rich missions for vendor bought items, if at all. I haven't measured whether they are worth it but they appear to be. Still, it's time spent for a slight discount when you could be getting better materials.
Edit: Also, I think you should not bother with the earlier vendor bought materials because they cost pennies. Whether you do it for bulk missions of rich yield items that cost 250+ credits per item is up to the individual.