**gcc 4.5.2, Score: 8579.15**

Original file, 29 bytes, compiles clean (a.cpp):

    #if 0
    #include"a.cpp"
    #endif

Modified file, 30 bytes:

    #iff 0
    #include"a.cpp"
    #endif

Errors:

    $ gcc -c a.cpp 2>&1 | wc -c
    1286873

Score:

1286873 / (30 * 5) = 8579.15