Cost of a basic wing-leveler in a an experimental type, the kit would be about US$2,000 - plus your own time to install it.
If you want fancy GPS steering, alt hold etc - assuming ne has the GPS already (you can feed from some handhelds) say double it.
The time if billed would likely run to about $15,000 - but if one has an experimental I assume one would do it oneself.
Only last month the FAA approved some low cost AP systems for 172/182, other types soon to be approved including the PA28 - the same month I spent $10,000 getting the ones in my PA28 refurbed
