echo "col1,col2,col3,col4
1,2,3,4
5,6,7,8" | \
awk 'BEGIN{FS = OFS = ","} {$3 = $3 FS (NR == 1 ? "budget" : "true")}1'
ZWNobyAiY29sMSxjb2wyLGNvbDMsY29sNAoxLDIsMyw0CjUsNiw3LDgiIHwgXAphd2sgJ0JFR0lOe0ZTID0gT0ZTID0gIiwifSB7JDMgPSAkMyBGUyAoTlIgPT0gMSA/ICJidWRnZXQiIDogInRydWUiKX0xJw==
col1,col2,col3,budget,col4
1,2,3,true,4
5,6,7,true,8