Sometimes a person will add "I'll say that much" to the end of a statement to indicate that the rest of your sentence is a gross understatement.
For example, after watching a very sad, depressing film like Schindler's List or The Color Purple, a critic might begin his review by saying "Well, it wasn't the feel-good movie of the year, I'll say that much."
Or "I'll say that much" might be added to a sentence as a humorous way to add some sort of positive "spin" or optimism to an overall negative situation:
A judge commenting on a particularly bad, off-key singer during auditions for a talent competition:
"He was dressed very well, I'll say that much."