Just one more addition, while not domain hacks per se (I think), I like the idea of using the extension to make a word speakable, provided that the name and extension both suggest reasonable value. Like a super great example would be e(.)co that sold for $81k a number of years ago. A few other great names along these lines are ban(.)co that sold for $24.5k in 2010 and magnifi(.)co that sold for $5k this year. I think co works well for these, but a few other extensions do as well. Interestingly ban(.)co does not resolve currently. A waste of a great digital asset!
When the combination is speakable, spellable and brandable it is great, and if it also is a word, or near word, in some language is another bonus. I acquired editi(.)co last fall as for me with the letter i added it made it more speakable, as well as of course making it into "edit ico" with possible offerings in editing windows ico icons or in initial coin offering use, possibly.
While the auction is long over, Sedo's domain auction earlier this year has some good general guidance - the
link is here.
In August there was a time limited promotion at Undeveloped where they reduced prices on many hacks for a short period. Does anyone know if they reported, in aggregate or individually, what sold? I know Undeveloped do not normally release sales, but would be interesting to see how many sold.
Of course many of the best hacks selling are at Park.io.