I can't imagine baking baking soda in an oven is cheaper than just buying washing soda? They're both sold in similar size bags (1kg) for similar prices in my area (€9-€10). Seems like a waste of energy to buy the wrong type of carbonate.
also, soooo many people will just ignore power usage bills, which can be quite substantial in some areas. Running my oven for an hour straight is not exactly cheap
It might depend on the size of the household, and the volume produced at any one time. If you make a gigantic batch that will last 1+ year(s), it might be a cost-saver, but who knows?