Short answer: the cheapest are cards with no annual fee — several Sri Lankan issuers offer lifetime or waived annual fees on specific cards. Examples (verify eligibility and current terms before applying):

• LOLC Savi Credit Card — advertised as a lifetime free card (no annual or joining fee). (lofc.lk)
  Note: marketed to certain customer segments; check LOLC’s site for eligibility and full terms. (lofc.lk)

• Bank of Ceylon (BOC) “Special” cards (e.g., State Sector / Doctors’ Card) — list lifetime/no annual fee for qualifying professionals. If you’re eligible (e.g., medical officers, certain public servants) these are effectively free. (boc.lk)

• Several mainstream cards waive the first-year annual fee (e.g., Commercial Bank Anagi shows joining fee and first-year annual fee waived). That makes them the cheapest in year one — subsequent years may have a fee. (combank.lk)

• Some cards (examples listed on aggregator sites) explicitly show zero annual fee options or very low annual fees (and some Platinum/Signature tiers charge higher fees). Compare tariffs on bank pages or an aggregator. (seylan.lk)

Why I can’t pick a single “most affordable” card for everyone:

• “Most affordable” depends on eligibility (some no-fee cards target doctors, government employees or existing customers), on whether you value first-year waivers vs lifetime no-fee, and on benefits you might want (cashback, travel insurance, lounge access). Tariffs also change, so the lowest-fee product can vary over time. (lofc.lk)

If you want, I can:

• Compare 3–5 no-fee / lowest-fee cards side-by-side based on your profile (income, profession, whether you want lifetime free vs first-year free).
• Or fetch the up-to-date annual-fee tariff pages from specific banks you name and confirm current fees (I’ll pull the bank pages and cite them).