Obviously money plays a part in identifying the pool you get to select from, there would be some guys who just aren't on particular club's radars.
Cairns has always had to shop in the bargain basement, and have often done very well. Say a team like Adelaide spent big to retain Randle (money well spent) but possibly didn't have as much to splash around on others.
With the new local marquee rules, many teams picked up at least one local marquee, on decent coin, so again the money left in the pot for imports may have been less.
Sydney went nuts early, secured a lot of very expensive local talent, and have subsequently decided not to spend as much on the imports.
Even Perth, who dropped their local marquee, would be paying decent coin to Martin, Knight, and even Wagstaff, plus Prather would have required more to re-secure, so I'd suggest that initially their 2 new imports weren't top-shelf. Certainly their FIRST attempt to replace JJ was a real discount affair. I imagine with the streak on the line, they coughed up more for Cotton.
But all the above is only half the story. No team sets out to hire crap imports, they look for the best available within their budget. Sometimes you unearth a gem like Randle, Ennis, or even Prather, and sometimes you get a crab.
You'd probably figure somebody like Cotton, who'd been playing in Turkey, is a decent standard for NBL, but if you're grabbing somebody form the D-League, or straight outta college, then its a lucky dip.
Sometimes you get lucky like with Cotton. He was playing in Turkey, which is a decent league probably comparable to the NBL, and probably getting good coin. But he decided he didn't like the country and wasn't going back. Offered the choice between finishing the season in the NBL or the D-League (assuming he could actually get into the DL at this point) it would be a no-brainier to come to Australia. But otherwise the NBL isn't always a destination of choice. It obviously pays more than the D-League, but doesn't offer the easy access to the NBA, and for players of a certain quality, they can probably get more money in Europe or similar. There's probably a lot of guys in the D-L who would come, like Andre Ingram, but they're going to be pretty average.