When Jesus asks Philip where bread can be bought for the crowd, Philip answers in monetary terms: two hundred denarii would not provide enough for everyone to receive even a little.
The figure is meant to make the ordinary economic solution look daunting.
The denarius was a common Roman silver coin and appears in Gospel contexts as a representative day's wage for a laborer, though actual wages varied by occupation, region, and circumstance. Two hundred denarii therefore represented a very substantial amount for ordinary people.
Philip is not necessarily presenting a precise shopping calculation. His estimate communicates the practical impossibility of feeding such a multitude from the disciples' available resources.
The ancient value of the coin helps recover the force of Philip's response without converting it into a misleading modern dollar amount. The disciples face a problem beyond their normal purchasing power.
John then contrasts that economic calculation with Jesus' provision, moving the narrative from what money cannot accomplish to the abundance of the sign.