Calculating the current value (GB pounds) of sums expressed in Egyptian piastres (1821)

It is often difficult to understand the current value of a sum of money spent in the past.
I have some data that might be useful for scholars who deal with 19th century explorers in Egypt and Nubia, and with expenses expressed in Egyptian piasters and British pounds.
In my case, I was trying to understand the value of all the expenses occurred to William John Bankes in sending around Linant de Bellefonds and Ricci in Egypt (Siwa and Sinai included) and Sennar.
Some receipts of the expenses are fortunately conserved among Bankes’ correspondence in Dorset. Letters and receipts by Linant and Ricci express the total expenses in a sum roughly near 50.000 Egyptian piasters (1).
A letter by Henry Salt to Bankes clarifies the exchange rate between Egyptian piaster and pound sterling: “the piastres are at 52 per pound sterling” (2). Then 50.000 piasters were at the time worth around 961 £.
Using this wonderful resource it is possible to convert a sum to its current value: OFFICER L.H., Purchasing Power of British Pounds from 1264 to Present, MeasuringWorth, 2011;
URL: http://www.measuringworth.com/ppoweruk/
It works considering the retail price index (RPI), which “shows the cost of goods and services purchased by a typical household in one period relative to a base period”.
The calculator can also give another result, according to the Average earnings, which are “a logical measure for computing relative value of wages, salaries, or other income or wealth”.
As the expenses given by Linant and Ricci were not the correspondent of a full salary, but rather an account of expenses for living and moving (probably camel and boat fees, food, water, gunpowder, presents, etc.), the RPI index is more appropriate to calculate the current value.

Here we are: 961 £ of 1821 are 69.300 £ of 2009!
A very large sum indeed.


More data:
Knowing that in 1820, 1 dollar (Maria Theresa thaler) was worth 15 piastres (3):
Ricci in his account states in 1821 that a slave was worth 10 dollars in Sennar and 90 in Cairo. That means 150 piastres and 1350 piastres. The exchange rate of piastres/GB pound as said was 52 piastres for 1 pound. The slaves were then worth 2.88 £ and 25.96 £ (GB pound in 1821). These sums today are the correspondent of 171 £ and 1830 £.

Ricci also says that a pair of shoes in Cairo was purchased for only 3 piasters, while in Halfaya (Sennar) it was for 35 piasters. This is 0.057 and 0.67 GB pounds of 1821. Nowadays they are 1.50 £ and 20.10 £

NOTES
(1) DORSET HISTORY CENTRE, D/BKL HJ 1/197, Memorandum from A.M. Linant de Bellefonds to W.J. Bankes, around 1822; HJ 1/198, Copies of payment receipt by A. Ricci, Cairo, 15 June 1821, 15 April 1822.
(2) DORSET HISTORY CENTRE, D/BKL HJ 1/153, Letter from H. Salt to W.J. Bankes, Cairo, 2 May 1821.
(3) BJØRKELO A., Prelude to the Mahdiyya: Peasants and Traders in the Shendi Region, 1821-1885, Cambridge University Press, Cambridge 1989, p. 107.