You can threshold to find all elements less than a certain number:
lowAmounts = amount < 10; % Logical vector - use find() if you want actual indexes.
Or you can use findpeaks() in the signal Processing Toolbox to find valleys in your data
[peakValues, locations] = findpeaks(-amount);
Or you can use the diff() function to find big changes from one element to the next
differences = diff(amount);
bigDrops = differences < -15; % or whatever value you want.
They're slightly different, so it just depends on what you want exactly.
