Is there a Steam API for apps, reviews and prices?

Yes. Social Fetch has nineteen Steam routes under /v1/steam: app details, a batch app lookup, store search, catalogue browse, suggestions, reviews, review totals, review history, player counts, news, achievements, community hubs, charts, tags, featured items, market search, market price, groups and the hardware survey. Authenticate with your Social Fetch API key in x-api-key.