In 1981 you could buy a 5MB Seagate
ST-506 drive for about $1500.
200 of those would equal a GB, so 200*$1500 = $300,000
1000 of those would equal a TB, so $300,000,000. Yep,
300 million dollars. (to adjust for inflation you'd have to at least double that figure)
Or if you use
Moore's Law, you can estimate that prices would halve every 18 months. Over thirty years, that's 20 18-month periods, 2^20 is about a million (in other words, double something 20 times and you'll get about a million). Multiply a million times the current cost of $100 and you get 100 million.
Either way it's hundreds of millions of dollars, roughly a million times the current cost of $100.
Similarly, today you can get 8GB of RAM for about $40. In 1981 you could buy 64KB for $550. So today you can buy 100,000 times the memory for 1/10th the cost. Again about a
million times more (cue Dr Evil).
Mxy, Lucky and 360 all did very well. I'll have to give this one to
three60 since his answer was around $100 million and he name-dropped Gordon Moore.