How far does eardrum move?
Tympanic displacement measurement (TMD) is a well studied field using hi-tech tools (i.e. stroboscopic holography), and complex units:
Vm = volume displacement in nl nanoLiters.
μm/pa (UDTF) = Linear tympanic membrane displacement is known as the umbo displacement transfer function.

The graph on the left gives you a value of 0.8-0.6 = 0.2 microns:


The graph above can be seen as resonances in an images sequence of stroboscopic holograms of tympanum (cool name to call your band) https://i.stack.imgur.com/fD6E9.jpg

According to this graph a 94dB sound wave gives you 0.1 micron displacement and a 100dB sine wave gives you 2pa, 0.2 microns.
https://i.stack.imgur.com/O7nQL.jpg

The first research document on google gave me a volume displacement from -120 to -700 nl. 

1000 nano Liter = 1mm3 

So the TMD volume displacement when very loud can be 1-2mm3, and eardrum rupture probably occurs from 10 to 100 mm3.

There are even muscles in the ear that can modify the way that you hear, i.e. the stapedius muscle which protects against low fequencies, and perhaps which can focus your eardrum elasticity modulus to let hear different frequencies, perhaps conciously? fascinating, I don't know.

See also pages describing elastic measurement of the eardrum, energy transfer by the tympanum by the cone, through these two google search phrasings: "eardrum response measurement" and "tympanic membrane displacement measurement"

Here is research comparing the eardrum to microphone design which is written easily.