With their monthly subscriptions these places let people download full resolution (XXL) images for $0.33/image. iStock on the other hand charges on average $6/full resolution image, even on a subscription account (as far as my math works out).

Doesn't that seem ridiculous? Do most of your sales come from these low end purchases? I wouldn't mind if the small or web sized images were being sold at that rate but 6000x4000 images for $0.25 a pop seems like robbery.

Welcome to Microstock world!
If you think that you can sell your files for more $$$ somewhere else (Alamy, Getty FAA.....) just leave it.

...For people who make a reasonable amount each month - is it mostly subscription or on demand revenue?

I make what I think is a reasonable amount at SS each month - four figures. So far this month, my income from subscription DLs is slightly more than the total from on-demand and and single-and-other DLs.

an unusual but very pleasant month at SS with only 34% subs

all time 61% subs

Every month I make more on non sub sales than I do on sub sales at SS.   Dt have a much higher rpd than any of the other big 4 sites despite getting quite a few subs there.

To answer one of your earlier questions:

I sell mainly subscription, but the on-demand ones make up about 40% of my income, so ....

I sometimes swallow hard on getting 19c for an image, but considering that it's a license and not a tangible, I suppose it's economies of scale that determines my happiness.