I used fish ferts until all my plants started wanting to bud up.. Then switched to tiger bloom, full strength. Worked really good IMO, You are right tho, they are pricey, if your growing less than 40 plants it wont set you back too much.. Either way I'd suggest seeing if you have a local wholesale nursery product warehouse, we have one in the area, they have 20-50lb bags of organic ferts and others that work just as well but they're like 1/10th of the price for 4times as much. Most people don't know about them since nurseries are mainly the guys doin business with em. Good luck.
Brandon