A ‘little sum (of money)’ could work, but I think ‘nest egg’ is used in reference to savings for the future, which the sentence alludes to later on:
“A nest egg is a substantial sum of money or other assets that have been saved or invested for a specific purpose. Such assets are generally earmarked for longer-term objectives, the most common being retirement, buying a home, and education.
The term can also refer to money kept aside as a reserve to deal with unexpected emergencies such as a medical problem or urgent housing repairs. “Nest egg” has been used to refer to savings since the late 17th century.”
https://www.investopedia.com/terms/n/nestegg.asp Then, I think sizeable, decent or significant are more used with ‘nest egg’ than respectable, considerable, etc.
My 2 cents, of course :)