If you are going for the cheapest then the TCI led strips from "the estonia guy on eBay" are impossible to beat. If he still has some for sale and if shipping isn;t an isssue where you live.
Your brief is missing some details. I'd guess a single Citizen 1825 COB running at max rated power would be one of the cheapest ways to get a lot of light. It's not very efficient though.
So you'd need to indicate what efficiency/efficacy you are looking for. Also, what are the intended dimensions of the grow area.