*Boots up SSW*
C3 Slave: 250,000 C-Bills
C3 Slave, Boosted: 500,000 C-Bills
Angel ECM: 750,000 C-Bills
So...
C3 Slave, Boosted (500k CBills) costs half that of the C3 Slave + Angel ECM combo (1 million C-Bills).
There's your logic right there.
Interestingly, there's a 1.5 million C-Bill difference between the Vanilla Master and the Boosted Master, so you WOULD get C-Bill Savings from a C3 Master + Angel ECM combo. But the C3 Boosted Master is 1 ton lighter and 1 crit smaller than the Master + Angel Combo, so what you're saving in C-Bills is getting made up in tonnage.
OTOH, if you field a mech with double Masters installed, 2 C3 Masters with 1 Angel ECM would be equal to the tonnage and crits of 2 C3 Boosted Masters, but cost far less money wise. And it'd be easier to find places to put 2 C3 Masters and 1 Angel ECM than it would be to find two open spots for 6 crit C3 Boosted Masters.
And one more thing: do ECM Bubbles disrupt C3 links if they're merely blocking line of sight between two mechs linked together when the mechs themselves aren't inside the ECM bubble? I can't recall if that's the case or not. If it is, ECCM isn't going to help keep that link up, but Boosted C3 will.