Besides the additional labor, materials and packaging, solutions require additional QC procedures. For comparison, to analyze a powder product, you only need to submit a single sample for analysis, even if the batch is 1000+grams, provided the batch is uniform and the sample is representative of the whole.
Solutions however cannot be aggregated into a single sample because that would tell us nothing about the consistency of individual bottles. So to verify 1000 bottles of solution you would need to test each one.
Instead we follow the industry standard practice of random sampling. Each lot of solution is sampled and we sample from different stages of the fill process to confirm there is no anomaly at any stage. But this can still be several tests per 1000 bottles and each analysis costs $150-350.