40 sous = 2 francs, or about $15. The conversion is a little screwy though, because physical commodities (food, clothing, etc) were much more expensive compared to, say, rent. So a franc could be worth anywhere between five and thirty present-day dollars.
For comparison: by law, a loaf of bread cost 1 sou; a skilled laborer like Feuilly made 3 francs (60 sous) a day; Marius, while poor, made about 700 francs a year, of which 30 went to his rent.
[Dieu] entend ta voix, ô fille des hommes! aussi bien que celle des constellations; car rien n'est petit pour celui devant lequel rien n'est grand.
- George Sand, Les sept cordes de la lyre