Ask HN: How much does it really cost to use Plaid?

I've been building an app that uses Plaid (https://plaid.com) in order to pull transactions from bank accounts on behalf of users, but so far I've only been using the sandbox API (free plan) for development. The next plan up from free costs $500/mo + "usage-based" fees, but I haven't been able to track down any quantitative dollar amount for the "usage-based" fees. I'm curious if anyone has any experience paying for Plaid's "usage-based" fees, and if so, what those realistically might be?