How many prints can I get out of a full set of ink cartridges? How much ink 'coverage' can I get out of a set of inks? What is the 'cost' of ink per print?

Depends,

Ink usage is determined by the Media Profiles and the images being printed.

Some media will accept more ink and that is reflected in the profile.
Some images have more 'color' than others.

You can find the ink cost study performed by Mutoh here:
http://www.mutoh.com/pdf/VJ12041604InkCostStudy.pdf

If  you look at the two examples:

You have 6.09 mils for 11.25 sq.ft. which works out to 0.541 mil per sq.ft  for one image.
And 13.68 mils of ink for 11.25 sq.ft. which works out to 1.216 mil per sq.ft.  for the other image.

which averages out to 0.879 mil per sq.ft. for this Glossy Vinyl Profile on Vinyl.

So for 880ml of ink (one set of 4 220ml), roughly you should get 1000 sq.ft. of prints.

These use two ISO standard images for evaluation.

Your actual ink use will depend on your image, your profile and the type of color managment that you use.
Ultimately, the PROFILE determines the number of droplets of each of the CMYK colors is used to achieve the printed output.

Most RIPs allow you to estimate the cost based on the number and size of drops calculated by the RIP for each and every print, based on the profile used, usually by inputting your ink cost into a 'calculator'. 

Coverage is another issue:

There is 100% coverage of a single color, i.e. Cyan,
There is also 100% coverage of all colors, i.e. rich black which could be 100% C + 100% M + 100% Y + 100% K = 400% ink, although most profiles will not put down this much ink.

So what is 50% coverage?
On what media?
Using what profile?

The variations are too numerous to provide even a rough estimate, that is the reason we use ISO standardized images and give only estimations for ink costs.

The best method to optimize ink usage is to have custom profiles written for your machine for the materials that you use, use good color management techniques and properly maintain your printer.