Throughout the 1820s the major currencies were fairly stable against each other. The pound sterling was worth about 5 dollars (American or Spanish), 25 francs and just over 100 Russian roubles.
The Turkish piastre however, which was also the Greek currency, was in sharp decline during this period. A few years before 1821 there were 20 piastres to the pound, in 1824 when Byron was in Greece the rate was 50/£, and by 1832 it had fallen to 86½. So the piastre had lost three-quarters of its international value in a couple of decades.
What would currencies of that time be worth today? Precision is impossible, not least because relative prices have changed. For example, in Greece at that time housing was cheap and transport expensive: you could rent a good house for a year for the price of three horses. Also labour was cheap and livestock expensive: a turkey would cost a labourer four days’ wages. However, it is safe to say that the pound sterling in Greece then – a strong currency in an impoverished country – had the buying power of at least £100 today.