I don't think there's a best practice, similar to how much you should charge for your B2B SaaS company, cause it really depends on the market size, cost to deliver, and perceived monetary value you're offering via your API.

That being said, the way I've baselined any pricing for my products has been to look at 100 companies that share similar customers and pricing model and either look on their website or email their sales team and ask directly.

Even if you get 10% response that's enough to get you started.