TLH pointers:
1. Your losses will off-set any capital gains you have first - so you're saving 0%/15%/20%/23%+/Marginal Rate + any state taxes depending on the types of any gains you might have and your income in the year you have them.
2. Once that is done, if you still have losses, you can off-set up to $3K of other income.
3. After that, it carries forward to future years to offset capital gains and up to $3K per year of other regular income.
So, you're paying 45% marginal per your post. If you have no capital gains to offset, then this year, you're off-setting $3K of income which saves you 45% of that in taxes - $1350. You paid $2100 this year to save $1350 in taxes this year. Yes, you've got a bank of losses to use in future years, but if you keep your money at betterment, you've also got that .15% racking up every year as well.
But even that's not the full story - you could tax-loss harvest on your own without paying betterment's fees. You might not capture quite so much in losses, but until you realize substantial capital gains, you're only able to save taxes on the losses in a trickle anyway.
How long are you earning $400K / year? At that income your capital gains will be taxed pretty heavily, whether long-term or short term, so offsetting those has quite a bit of value but if you back-off on the income in the future, your tax-rate on long-term capital gains could be as low as 0% + state taxes. You might never see the full benefit of the losses you've harvested.