Cost of business trip small? That’s about $100 too much (6)
I believe the answer is:
excess
'too much' is the definition.
(excess of something means too much of it)
'cost of business trip small? that's about $100' is the wordplay.
I cannot quite understand how this works, but
'small?' could be 's' (abbreviation - e.g. clothes size) and 's' is found in the answer.
'about' could be 'c' (abbreviation for circa) and 'c' is found within the answer.
The remaining letters 'exes' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for excess that I've seen before include "Too much - going too far" , "Intemperance" , "Glut; surplus" , "Glut, superfluity" , "Over and above what's needed" .)