The Egyptian pound (or "guinea" in Arabic) in this period was pegged to the pound sterling at 1 GBP to .975 EGP. A piaster is 1/100th of an Egyptian pound.

Measuring the historical value of a currency is complicated. In 1938, the average British wage earner earned 126 pounds sterling.

Workers today earn significantly more than the average wage earner in the UK in 1938, but just proportionally the equivalent for an American median wage earner today ($52,000), 5/100s of a pound would be $20.

This is then drastically complicated by the fact that, as far as I'm aware, we don't have any great way to compare purchasing power or local costs (except to note in the novel the cost of the stuff that is referred to as costing 5 piasters in the first place). So while Egyptian workers were earning drastically less than their British counterparts, their cost of living was likewise drastically lower. The price of staples like bread (and by extension all food) in particular have historically been lowered significantly by subsidies. 160 grams of bread in the 1980s might only cost 1 piaster while a 400 gram loaf of bread in the US today might cost $2-3.

This is not a great answer to your question, but the basic answer is that thinking about this in the terms that the book offers is probably the best way to think about this in terms of what the characters in the book are buying for that price in that purchasing power, and in terms of whether or not they regard it as a routine or an extravagant purchase based on their social standing or income.