Let me say first that I do not work with a firm that provides US tax return completion services.
But our US clients do ask for referrals to such firms.
If a client asked us for a catch-up and compliance service of the kind you have mentioned, I would create a shortlist from those individuals who had worked for the larger firms and were now out on their own, or in small partnerships. I would expect them to charge less, but be just as good. About 600 per year is the rate for US and UK returns and the interaction. If you have a batch of three years and the pattern of income / claims is common then I would expect a small discount. As usual they look for the long term win-win: they like to select those clients who value what they do and who are profitable.
Hope that helps.