About $1k to grow a pound (first setup including lights nutes etc, if you know how to shop and don't use ever snake oil out there).
At $5k/lb that works out to a 400%profit, 100%profit on a $1K investment = $1k, 200%=$2K, 300%=$3K, 400%=$4K, so $5K minus your initial $1k investment=$4k=400% profit. This is kinda simplified, a 20K watt grow op will have a larger startup but the profit margin, factored over a few grows or years will be about the same,assuming the the final yield to initial investment ratio is about the same.
Does that make sense? I think I'm thinking this out right.