I wouldn't worry too much about cable reuse. You can find them for $10 on monoprice.
Displayport isn't exactly one thing. There are different specifications.
See the wiki. More to the point, anytime you have an identical connector on your machine and the display, you should get a cable that matches at both ends. What do you think you could possibly gain through a conversion on one end? Anytime we're talking about cables, they go by lowest common denominator. If you needed to support an older machine with a new monitor and had to use a displayport to dvi, you would lose any real advantage of displayport apart from connectivity. The one thing you should not use is a thunderbolt cable unless you're connecting to a thunderbolt display. I don't know of any other than the one sold by Apple. Also just for reference, thunderbolt 2 and some displayport 1.2 features haven't made it to the mini.