Mwave offers the Syba PCI-Express 2-Port Serial Card, model no. SD-PEX15022, for $19.99 with free shipping. (Fry's charges the same with in-store pickup.) That's the lowest total price we could find by $2, outside of the mentions below. It feature two 9-pin RS-232 serial ports, single-lane PCI express x1 interface, 2.5Gb/s throughput, and transfer data rates up to 250Kb/s.
A close price: Newegg has it for $20.99 with free shipping, while Amazon also charges the same with free shipping for Prime members.