Bode diagram calculating for random frequency
A roll-off of 20 dB per decade means that the gain reduces by ten for an increase in frequency of ten. This means that at 100 Hz the gain is 10,000 (as you said) and at 200 Hz it is 5,000 or 74 dB i.e. 6 dB lower than 80 dB. 

At 10 kHz the gain in decibels is 40 dB or 100 and, at 20 kHz the gain is half of this at 50 (or 34 dB).