Here's a choice piece of code we have in production along the lines of what you suggest:

    if prorated_amount > (cost_per_month * 2)
      raise "critical! we were about to prorate-charge someone #{prorated_amount}"
    end

It's never been triggered, but this is probably the only place in our system where we're actually responsible for an exact amount that ends up on an invoice, so having a reasonable upper limit seems like a no-brainer.