Business Lunch near Gare de Lyon
"Nice but not too nice so le Train Bleu is out."
Why "not too nice"? Otherwise the Train Bleu would fit your budget. And if you order "defensively", sticking to classic non-tricky dishes, you can even eat not badly.
If you resolutely do not want a resto that looks too good, Le Quincy is not far. An 8 minute walk from Gare de Lyon according to google map.