Not cheap, but it is God damn worth it. The secret is that you only need to innoculate once at the beginning. I suppose you could innoculate at flower if you don't believe that microbial life isn't doing everything it can to stay alive.
I consider these more as BuildASoil products, but these are great mycorrhizae and microbial inoculants.
Each product has specific microbes which help to mobilize nutrients during the various phases of growth (Veg/Bloom).