In my opinion there is no need for a salary cap in cycling.
I understand this issue because you have 18 WorldTour teams, competing on highest level and the WorldTour System is a little bit similar to the franchise systems of the sport league system in the US sports (NBA, NFL, MLB...), so far so good...
But actually cycling is a sport of specialization. (Climbers, Sprinters, Cobblestone and so on...). and so Teams put there focus on one or two terrains. For example Sky is now focused on GrandTour and OmegaPharmaQuickstep on spring classics. So every team is trying to build up a capaple team to reach their objectives in its specialization. But all of this tries have their limit. For example a Tour de France contender never would go at the moment to Sky, where it would be uncertain to be the number one, so he would choose an other team.
Besides history has shown that all of these experiments of these "super-teams", teams that came out of nothing with a big financial background, failed more or less.... Astana 2009, Radioshack & Leopard 2011 plus the merger this year so far or the Sky & Cav issue this year.
So a star rider is always looking to have all advantages and support on his side and dont want to share this with a rival of the same terrain in one team.
And all these facts will arrange for cycling that there will be allways enough contenders and competition and no concentration of all potencials on one ore two teams