Could you use the free tier for regression testing a subset? Like up to 1M either first, last or random sample? Or do the datasets themselves have to be prefiltered down to 1M results?
For the majority of diffs we see with sampling applied, sample sizes are <1M rows (more is often impractical in terms of information gain for higher compute costs) especially if your goal is to assess the magnitude of the difference as opposed to get every single diverging row.