BOFH version

#!/usr/bin/perl

print join(";", qw(2015 jan feb mar apr may jun jul aug sep oct nov de
+c));

while(<DATA>){
    chomp;
    print ( s/^(\d+)\/\d+\s+// ? (";" x ($1-$c)).";$_" 
    : ($c=0) || "\n$_" ) && ($c=$c?$1+1:1);
}
print "\n";


__DATA__
CUSTA
1/2015 100
10/2015 1,000
12/2015 1,000
CUSTB
02/2015 200
10/2015 1,000
12/2015 100
[download]

Output:

2015;jan;feb;mar;apr;may;jun;jul;aug;sep;oct;nov;dec
CUSTA;100;;;;;;;;;1,000;;1,000
CUSTB;;200;;;;;;;;1,000;;100
[download]

edit: small typo