I suggest calculating the stats each new building or upgrade would give you divided by its price. Include the land price for lvl 1 buildings. This will let you increase your stats by the most for the least gold. Each time you get a new land, the price of the next goes up so at some point it'll be more cost effective to upgrade your buildings to level 2 than to build a new one. Eventually, (I think) the same will happen with upgrading to level 3. I haven't built on the HF, but I do know that this will get you higher stats for cheaper at first, then growth will slow later.