(one) will be lucky

That is very unlikely to happen or be the case. The phrase is used sarcastically or with a sense of resignation. And just who is going to pay me back for this thing now? I'll be lucky if I get 50 bucks for it at a pawn shop! If you're calling a government office, you'll be lucky to get a live person within 30 minutes. I've heard Mrs. Smith's exams are notoriously hard—like, we'll be lucky to even pass, that hard.

you, he, etc. will be lucky (or should be so lucky)

used to say that someone's wishes or expectations are unlikely to be fulfilled.

ˈyou’ll, ˈhe’ll, etc. be lucky

(also ˈyou, ˈhe, etc. should be so lucky) (spoken) what you expect or hope for is unlikely to happen: You were hoping I’d come and collect you from the airport after midnight? You’ll be lucky! Try a taxi! ♢ ‘Come and see us if you are ever in Australia.’ ‘I should be so lucky.’