Cheap doesn't do it,
Do you have the Collins [two green illuminated buttons] or FGS-70 [switch up for AP/YD down for YD] autopilot? It makes a big difference.
Hopefully you have the Collins in which case your better bet is the Honeywell right-side altimeter that basically has an ADC in the back. The cost is still going to depend on the integrity of the existing installation.
Dont forget you'll also need crew training and approval to operate from your state of registry.