I can't answer your question directly but I can help to put the figure into some sort of context.
Puccini received 120,000 lire from the Metropolitan just for the first night royalties alone for 'The Girl of The Golden West' (so he certainly wouldn't miss the odd 5000 lire!). At the time this equated to US$22,800. Some simple mathematics means that 5000 lire would have equated to US$950 which at the time wasn't a small amount but neither was it enough to retire on! The site I got this information from also states that 120,000 lire represented more than 1,600 times the monthly pension his mother had raised her children on. Some more simple mathematics shows that his mother's annual income must have been something less than 900 lire so this gives us some idea of the value of 5000 lire.
Source:
http://www.americanheritage.com/articles/magazine/ah/195 9/3/1959_3_52.shtml
Strangely, none of the sites I've looked at make any reference to Puccini paying his mistress off in 1910. But it was at about this time that his family were sued by the family of Doria Manfredi (his servant who committed suicide after being falsely accused by Puccini's wife - and former mistress - Elvira of being Puccini's mistress). Perhaps the historical records have got a little muddled?
Sources:
http://www.litencyc.com/php/speople.php?rec=true&UID =3659
http://www.geocities.com/SoHo/Cafe/1614/Music/Eru/puccin i.htm
Hoping this helps,
Chris