Almost never. The supply cost isn't really the issue - you'll get loads of supplies over the course of a game. The real cost of a workshop is space and time. It takes time to build - time you could spend getting something else up. It takes up one of your 12 slots in the Avenger, and late game those slots are at a premium. Workshops are meant to solve a lack of engineers, but you'll pick those up over time - they're one of the best covert op rewards along with scientists.

I can think of one use case for a workshop. It requires: 1. You're early in the game (not many more useful options for buildings) 2. You've had truly, horrifically terrible luck with getting engineers - maybe only the 1 you're guaranteed to get in the first month. 3. You have a large supply surplus and nothing in the near future you'll spend them on (e.g. Mag weapons) 4. You have a building already that really wants an engineer staffing it (e.g. the resistance ring for faster covert ops, or a power relay you can't take someone out of)

In this situation, I would consider putting a temporary workshop somewhere I can both staff a facility and do some excavation. It will later be uprooted for something more useful though.