Short answer — for strictly lowest cost: Melio. Its Go (free) plan gives you a usable AP/payments workflow with up to 5 free ACH vendor payments per month; paid tiers start at about $22–$25/month if you need more volume. (meliopayments.com)

Quick summary of the cheapest practical choices (with pricing/features as published):

• Melio — cheapest entry point (free Go plan: 5 free ACH/month; Core ≈ $22–$25/month; Boost/Unlimited tiers if you need more ACH or users). Good if you primarily need to pay vendors (ACH/card/check) and want a very low-cost option. Caveats: the free plan is limited by free-ACH count; card and expedited options carry fees. (meliopayments.com)

• QuickBooks Online — Bill Pay feature: if you already pay for QuickBooks Online, the included Bill Pay Basic tier (or low-cost Bill Pay Premium/Elite add-ons) can be very economical (Basic includes 5 ACH/month; Premium/Elite add more free ACH or eliminate fees). Good if you already use QBO because it’s tightly integrated. Note: QBO subscription itself is a separate cost. (quickbooks.intuit.com)

• Zoho Books — has a free plan for very small/micro businesses but AP automation is limited; their BillPay add-on (for automated ACH/vendor payments, PO/bill workflows) is a separate paid add‑on (≈ $59/month). Good if you use Zoho ecosystem. (zoho.com)

• Bill.com / Stampli — full-featured AP automation (approval workflows, PO/3‑way matching, enterprise features). These give stronger automation but are materially more expensive (Bill.com plans start in the ~$45/user/month range and up; Stampli requires contacting sales). Choose these if you need heavier AP automation and control. (bill.com)

How to pick the cheapest right for your business

• If you just need to pay vendors and volume is low: start with Melio’s free plan. (meliopayments.com)
• If you already pay for QuickBooks Online: evaluate QuickBooks Bill Pay (it may be cheaper overall because of the integrated workflow). (quickbooks.intuit.com)
• If you need advanced AP automation (PO matching, multi-approver workflows, audits): budget for Bill.com or a similar provider. Those are more expensive but reduce manual work at scale. (bill.com)

Notes and caveats

• Vendors change pricing, promos, and free-tier limits often — the cited pages are current as published by the vendors. If you want, I can check any one vendor’s current live pricing and fees (for your exact expected monthly payment volume and preferred payment methods) and do a direct cost comparison for your situation.