Just pick a hand calculator and start adding prices. Make a scratch design of such a card. What it would require? High voltage ESD protection, certainly. PCI bus buffers, certainly. Custom drivers for Win98, WinME, WinNT/2K/XP, Linux, certainly. PCI bus slave hardware, yes. Quad layer PCB (we don't want to break other PCI hardware, do we?) yes. Microcontroller to drive the serial interface, probably. (or FPGA). Buffers, line drivers, decoupling, crystal and all that stuff needed to drive the microcontroller? Must go there. ETC ETC.

Present them with a nice 3 digit figure for parts for the custom PCI card, then compare it with alternatives.