Trump has operated dozens of million dollar corporations, so I think it silly to believe the guy doesn't know basic math! This is likely more a case of Trump just speaking metaphorical campaign talk, partly because he might think this is more along the lines that many people understand.
Obviously, you cannot cut the cost of something by 800% without it becoming free and your getting a refund or payment for using it! My guess is that when he says prices cut by 800%, what he is intending to relate is that costs have been cut to 1/8th their former cost.
Trump is a bit of a carnival man, not that I think he is stupid or intending to actually deceive, he just has a tendency to relate stories and put things in dramatic big tent ways!
The biggest ever! Everything is biggest, best, longest ever.