Ahrefs (SEO analytics): $99 / mo. I haven't splurged on this yet but likely will in the next month, SEO being so critical to my marketing strategy across all products.
Clickhouse (real-time analytics): $500 / mo. The natural next step for my stealth analytics product. Considering the cost, I'm holding off until usage requires the additional expense over a Postgres DB.
Scaling costs: We're approaching the limits of the basic tiers for Vercel, Neon, and Postmark. Soon enough the costs for those will start to increase with our usage.