I believe something like this would do it, at least up to 999,999.99, which seems likely it's good enough for a discount :). 
(you could also add another "(\d{1,3})?,?" in front to capture up to 999 million)

Discount Amount:\s*?(\d{1,3})?,?(\d{1,3})?(\.\d\d)

But let's break it down a bit:
The first part, "Discount Amount:", is anchoring text and helps you verify you're capturing the correct number.  This is followed by \s*, which means "any amount of whitespace".

Then we have "(\d{1,3})?", which says there is an optional set of 1-3 digits that we want to capture.  This would be capturing the thousands block (e.g., values between 1,000.00 and 999,999.99).

And an optional comma (",?") follows that, which means that if the value is less than 1000, the pattern will still match.

And then this pattern repeats again for the "ones block" (e.g., values between 1.00 and 999.99).

Except that it doesn't need the optional trailing comma.

And then finally we capture the "cents block" which, importantly, captures the decimal point (so we don't accidentally turn 1.00 into 100).