what does "lost a shilling and found a penny" mean?

I am translating a British story and I came across this expression

"you look like you lost a shilling and found a penny". I am not sure I understand what it means.

A shilling and a penny are two increments of currency, where a shilling is made up of many pence (the multiple for penny when it is a unit of currency). As it happens, the shilling is no longer in use in Britain, so this is somewhat of an archaic saying.

Essentially though, the saying implies that you've found something less valuable than what you lost — or simply, that you look disappointed about something.

A more modern version of the saying might go: "you look like you've lost a dollar and found a penny".

Prior to 'decimalisation' in February 1971, British currency followed the structure that had been used since pre-800 AD. This was as follows:

One pound (£1) = 20 shillings (denoted thus: 20s or 20/-)
One shilling (1/-) = 12 pence (12d) (singular penny) (the abbreviation d for penny/pence originated from the Roman denarius)
Thus there were 240 pence to the pound (strictly the pound sterling, so named because the value of the pound was originally equivalent to the value of one pound weight of sterling silver).

The penny was further subdivided into:
- 2 half-pennies (½d) (pronounced ha'penny (singular) or ha'pence (plural)); or
- 4 farthings (¼d)

Coins with values of one-quarter of a farthing (1/960th of £1) and one-third of a farthing were in use in some of the British colonies in the 19th century.

On decimalisation in 1971, the value of the pound was retained unchanged, but became subdivided into 100 new pence (abbreviation p) (the new has since been dropped).

The name shilling was dropped, but the then one shilling coin became worth 5 new pence. All pre-decimalisation coins of value sixpence (half shilling) or higher simply adopted their corresponding new values, and pre-decimalisation coins of lower value (which had no exact post-decimalisation equivalent) were withdraw and replaced by 'copper' coins of value ½p (since dropped), 1p & 2p (still in use, although coin size has changed).

So back to the original question:

to lose a shilling and find a penny

refers to the pre-decimalisation one-shilling and one-penny coins, and (as others have intimated) was an old British idiom indicating that you had lost something of value and found something of considerably lower value.

The pre-decimalisation shilling coins remained in use until 1991, but their new equivalent, a smaller five-(new)pence coin, continues in use. Sadly, the old one-penny coin is long deceased.

Finally, clarification of plural of penny (supplementing the answer from @Hannele):

The usual plural of penny - used when referring to it as a currency unit - is pence.
All British coins with values between 1p (one penny) and £1 (one pound) use the word pence in stating their value.

Another form of the plural is pennies, but this is normally used only when referring to multiples of one-penny coins: I have three pennies would be understood as meaning I have three one-penny coins.