/tmp$ cat test.awk
{
P=$3
PS=sprintf("%.2f", P)
for (i = 1; i <= NF; ++i) {
if (i != 1) printf FS
if (i != 3) { printf $i } else { printf PS }
}
printf "\n"
}
/tmp$ cat test.txt | awk -f test.awk -F,
09/28/2009,09:10:37,35.60,35.29,35.75,150
09/28/2009,09:30:06,35.48,35.34,35.64,14900
09/28/2009,09:30:06,35.49,35.37,35.61,100
09/28/2009,09:30:10,35.40,35.4,35.57,100
09/28/2009,09:30:10,35.48,35.43,35.53,200
09/28/2009,09:30:11,35.46,35.44,35.56,200
09/28/2009,09:30:14,35.57,35.49,35.57,100