gcc 4.5.2, Score: 8579.15 (or 14367.49 for filename "a.C", may update later)
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
Head and tail of error output:
a.cpp:1:2: error: invalid preprocessing directive #iff
In file included from a.cpp:2:0:
a.cpp:1:2: error: invalid preprocessing directive #iff
In file included from a.cpp:2:0,
from a.cpp:2:
a.cpp:1:2: error: invalid preprocessing directive #iff
In file included from a.cpp:2:0,
from a.cpp:2,
from a.cpp:2:
a.cpp:1:2: error: invalid preprocessing directive #iff
In file included from a.cpp:2:0,
from a.cpp:2,
from a.cpp:2,
from a.cpp:2:
a.cpp:1:2: error: invalid preprocessing directive #iff
In file included from a.cpp:2:0,
from a.cpp:2,
from a.cpp:2,
from a.cpp:2,
from a.cpp:2:
a.cpp:1:2: error: invalid preprocessing directive #iff
In file included from a.cpp:2:0,
from a.cpp:2,
from a.cpp:2,
from a.cpp:2,
from a.cpp:2,
from a.cpp:2:
... And so on, backing out with second error after max include depth:
a.cpp:3:2: error: #endif without #if
In file included from a.cpp:2:0,
from a.cpp:2,
from a.cpp:2,
from a.cpp:2,
from a.cpp:2:
a.cpp:3:2: error: #endif without #if
In file included from a.cpp:2:0,
from a.cpp:2,
from a.cpp:2,
from a.cpp:2:
a.cpp:3:2: error: #endif without #if
In file included from a.cpp:2:0,
from a.cpp:2,
from a.cpp:2:
a.cpp:3:2: error: #endif without #if
In file included from a.cpp:2:0,
from a.cpp:2:
a.cpp:3:2: error: #endif without #if
In file included from a.cpp:2:0:
a.cpp:3:2: error: #endif without #if
a.cpp:3:2: error: #endif without #if
Note: If .C ends up qualifying as a valid extension then score is 1206869 / (28 * 3) = 14367.49.