Global Search require three characters minimum.

Why on earth would you keep so many messages in your Inbox?

Thanks for the quick reply.

A 3 character minimum is an interesting feature! I also get no matches when I search for $82.81 which is in several emails.

Why clean up the inbox? One never knows when an old email might be useful and storage is virtually unlimited now.

I find it more productive to organize my mail by filing away according to context, topic or correspondent. Large collections of messages tend to be less efficient. No need to delete, just put away in an orderly fashion.

If all your stuff is in the one place, why do you need to use global search?

Thunderbird's global search is frequently criticized for several shortcomings; its non-specificity is a common cause for complaint. And like many simplistic search tools, it probably doesn't like numerals.

For challenging searches, I use regular expressions ("re" - see FiltaQuilla and Expression Search/Google Mail UI) and a Saved Search folder to aggregate all the required folders.

This search string will find me all mentions of a £ or a $ followed by an optional space, some digits and optionally, a decimal point and some more digits.

[£$] ?[0-9]+(\.[0-9]+)?

You could of course replace the wildcards by specific numeric values.

\$82\.81

should find that explicit $82.81 you were looking for. Note that both $ and the period have special meaning in re's so must be escaped by the backslash.