Example
Six months left in the year. COBRA $700/month, $500 left on your deductible: 6 × $700 + $500 = $4,700. Marketplace silver after credit $450/month with a $1,500 deductible you would reach: 6 × $450 + $1,500 = $4,200. The marketplace is $500 cheaper here, but if you were mid-treatment with doctors outside the new network, COBRA could still be the better choice.
The COBRA calculator runs this with your numbers and the marketplace calculator estimates the tax credit.
General education, not insurance, tax or legal advice. Figures from IRS and HHS publications; confirm with your plan documents.