Short for "decent purchase". Orginally used as druggie-slang/code for a good drug deal or drug quality, but recently has been used in response to any type of favorable purchase, or simply to replace the word "dece".
"Yeah, that coke was a dece purch"
"Lemee see that cellphone......damn, dece purch."
"Ok I'll come to your house later"
"Aiyt, dece purch"