Recently I was stuck a little on the problem, how to obtain URL of the Central administration web application. After little research I've found out 2 quite simple ways:
1. Using SPAdministrationWebApplication
Now you just have to determine host name and append port to it.
2. Using Registry
Registry.GetValue(@"HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Shared Tools\Web Server Extensions\12.0\WSS", "CentralAdministrationURL", "none")
Both cases working fine for me.