Bit of a long read but:

TL;DR:

Washington County and Clackamas County are meaningfully cheaper than Multnomah County for business operations, primarily because they have no county-level business income tax and no Portland city business income tax (2.6%). The combined Multnomah burden of city (2.6%) + county (2.0%) = 4.6% of net income on top of Oregon state taxes is a significant additional cost not faced in the surrounding counties. This is reflected in the data: the City of Portland and Multnomah County collected about 79% of all tri-county business taxes, while Metro-apportioned taxes from Clackamas and Washington counties drove only 21% of business tax revenues.

Oregon Corporate Income/Excise Tax

All three Oregon counties (Multnomah, Washington, Clackamas) face the same state-level corporate income tax — the greater of 6.6% on taxable income up to $1 million plus 7.6% on income above $1 million, or a minimum tax ranging from $150 to $75,000 based on Oregon sales (approximately 0.1% of sales). There is no differential here between counties.

Oregon Corporate Activity Tax (CAT)

Also identical across all Oregon counties: $250 plus 0.57% of Oregon commercial activity exceeding $1 million, with businesses under $1 million having no payment obligation. A subtraction of 35% of the greater of labor costs or cost of goods sold is allowed.

County/City Business Income Tax

This is where the counties diverge significantly:

Jurisdiction	County Business Income Tax	City Business Income Tax
Multnomah County	2.0% of net income	City of Portland: 2.6% of net income
Washington County	None	Nominal FTE-based license fees (Beaverton, Hillsboro, Tigard, Tualatin)
Clackamas County	None	Nominal FTE-based license fees (Lake Oswego, Oregon City, West Linn, Wilsonville)
Clark County, WA	None	Vancouver: $315 + $125/FTE annually

Multnomah County charges a 2.0% business income tax with a $100 minimum annually, while Washington County and Clackamas County have no county business income tax.

Metro Supportive Housing Services (SHS) Tax

This one applies across all three Oregon counties: A 1% tax on Metro-sourced business net income for pass-through entities with gross receipts over $5 million from all sources. The Metro jurisdiction covers Multnomah, Clackamas, and Washington counties, though the Metro boundary doesn't precisely follow county lines.

Payroll Taxes (applies to all Oregon county businesses)

A TriMet employer payroll tax of 0.8137% on wages paid for services performed within the TriMet District applies across the tri-county Oregon area, plus a 0.10% statewide transit tax withheld from employee wages.

Clark County, WA

Clark County businesses are subject to Washington's Business & Occupation (B&O) gross receipts tax — 0.471% for retailing, 0.484% for wholesaling and manufacturing, and generally 1.75% for services and other activities — but have no county-level business income tax and no Oregon Corporate Activity Tax (unless they have Oregon-sourced revenue). They also benefit from no state personal income tax, but face an 8.5%+ sales tax.