Can I sell mushroom compost as a business to other growers?

Yes, supplying ready compost, often ready spawned compost, to other button mushroom growers is a real and useful business, because many small growers cannot make good compost themselves and would rather buy it ready, just as you once might have. It works best if you can produce consistent, well pasteurised, selective compost in volume, which usually points to short method tunnel composting for reliability. Your customers are the small and seasonal button mushroom growers in your region who want to skip the difficult composting step and go straight to cropping. The value you offer is consistency and freedom from contamination, so quality control is everything, since bad compost will fail your customers crops and your reputation. It is capital and skill heavy compared to just growing, needing composting infrastructure and know how. Because returns depend on scale, consistency and your customer base, I would not quote a fixed income. Validate local demand with nearby growers and your KVK, and get the composting process right first through training at DMR Solan or an SAU.