So I ran a monte carlo simulation (on portfolio viz) and the difference in monthly income, using the 3.5% rule, was 1860$ for the non case and 1808$ for the fee case. I used my current plan of 65k starter, 15 years, 100% stocks, 2k a month, and I modeled with Betterment's portfolio.
This is assuming Betterment itself has zero benefit which I think it does. I mean if you don't want to do Betterment fine, but you must have done it for a reason. I personally think the biggest reason is behavioral, as the platform is designed to get you investing more and to do so for the longer term in a way that actually works.