Then after that, yes you have to do the math on your marginal tax rate times your HSA contributions. The limit next year will be $6900, so after the $1500 and $150 that my insurer puts in I can contribute another $5250. That used to be in the 22% tax bracket, but under the new TCJA my marginal rate is lowered to 12%, though they took my deductions away so I pay it on more income. So my tax savings from the HSA are only 12% of $5250, or $630. It's not nothing.
Note that the decrease in income from HSA contributions will also increase your premium subsidy. This effect essentially adds a few percentage points to your tax bracket.
I'm looking into this myself for our first partial year of FIRE next year. I'm going to try and target an income just under 200% of the poverty level in order to qualify for the 87% cost sharing silver plans.
The cost sharing plan I'm looking at looks pretty nice:
$400/month premium for my wife and me after tax credit
Nominal copays for doctor visits or urgent care visits, no deductible involved for these.
$1,900 deductible per person that seems to only apply for hospitalization or surgery or exotic prescription drugs. $1,900 is also the out-of-pocket max, so everything's free after that.
On the other hand we could get the following bronze plan:
$150/month premium for my wife and me after tax credit
$4,750 per person deductible that applies to every non-preventive service.
20% coinsurance after that, up to the $6,550 out-of-pocket maximum
Ability to contribute to an HSA
The bronze plan saves $3,000 on premiums over the course of the year, but the tradeoff is a medical plan that basically does nothing until we rack up $4,750 in medical bills. At our expected income level we might not even be in the 10% bracket since a good chunk of our income will be coming from capital gains and dividends, so the immediate tax savings from HSA contributions will be pretty minimal.
Part of me thinks the rational thing to do is take the bronze plan. If we shove the $3,000 in premium difference into the HSA we'll probably come out ahead most years. Another part of me really likes the idea of being able to go to the doctor for $10 even if "it's probably nothing," and really limiting the amount of financial downside if we do need a bunch of medical treatment. Financially it's probably pretty close to a wash in the long run, with the bronze plan resulting in more cost variability from year to year.