If you make it an awk variable, it should just work.
outputfile="/temp/out.txt"
a=100
for i in 1 2 3 4 5
do
Actual=`awk -F"," '{x += $i} END {print x}' i=$i Trailer_Data_Details.csv`
echo $Actual,$a > $outputfile
done
# Your for-loop only takes the first 5 columns...
201,100
401,100
601,100
801,100
1001,100
Doing the whole thing just in awk, for every column:
awk -F"," '{
for( i = 1; i <= NF; i++ ) x += $i
}
END {
for( i = 1; i <= NF; i++ ) print x "," a
}' a=100
201,100
401,100
601,100
801,100
1001,100
1201,100
1401,100
1601,100
1801,100