I recently talked with someone at Guideline and asked them that same question: How can you offer a much lower cost than Vanguard, for example, for a small business 401K? The reps answer was that most of the investment companies, like Vanguard, use 3rd party services for many of the tasks that aren't directly related to managing the investment funds (what Vanguard does best), and the cost of those services from 3rd party companies is extremely high and gets passed on. With a company like Guideline, they do all that work in-house (record keeping, fiduciary responsibilities, etc). and leave the actual fund management to Vanguard (though Guideline does review the fund menu on regular basis).
I'm a newbie at all this so I don't understand the nuance of it all, but that explanation made sense to me.
Could you replicate it? Probably. But I would think that unless you're already an expert in this area it may be a tough hill to climb.