what's the difference in amount of ink used between print qualities?
Could it be that there is only difference in speed (lower quality- faster
,better-slower)
You are almost right. Ink consumption don't depends on printing resolution.
Most printers have an economy resolution (draft/economy), in this case the
consumption is of course lower. An example for an Epson printer, if you
think the Normal resolution consumption as 100 units:
Economy --> 50-60 units
Normal 360dpi --> 100 units
Fine 720dpi --> 110 units
SuperFine 1440dpi --> 115 units
Photo 1440dpi --> 115 units
Etc.
All above are a personal estimation for plain paper printing. If someone use
special paper settings (photo glossy, matte) the consumption is higher
(about 50%, in some cases double).
Consumption depends on how printer manufacturer design each printer model.
E.g. Generaly speaking HP and Epson printers consume less ink in
draft/economy mode than other makes.