I've never planted a GE. The math really isn't that good for planted GEs or planted GMs. Actually, the math isn't good for GMs no matter how they're used, which is a real shame.
Consider: working a manufactory entails an opportunity cost that must be deducted from the bonus hammers. The minimum value of the opportunity cost is 1
or 1
, which is what you could get by improving the tile normally instead of planting the manufactor. Considering that you're not likely to get a GE until wonders cost at least 300, you would need 100 turns of manufactory production to match the immediate investment. Now assume that instead of planting the GE, you just waited for 100 turns. At this point, the wonder value of the GE should be at least 500
and probably more like 1,250
. From this value, you would need to deduct gold maintenence, but I still can't see you ever getting a better hammer result with the planted GE versus an immediate wonder or waiting for a wonder. The exceptions are when you're desperate for units or a strategic resource.
Random aside: Custom Houses are total, unadulterated garbage. They need to be fixed. They're terrible beyond words.