You can throw bucket loads of money at any autopilot, the older they are the worse that might be, but it doesn't necessarily need to be like that. I know from experience.
I have been involved with a 50 year old autopilot that some shops wanted to replace and others were able to fix for a minute fraction of replacing it. By the way it had altitude preselect, envelope protection, and when coupled to an EFIS, GPSS.
You just need to make sure you get someone who is knowledgeable and methodical. In my opinion you need to take the aircraft to the shop and not just send the autopilot computer for testing/adjustment as the problem is quite possibly not the computer/controller.
Easy and cheap things to check on first are control cable and bridle cable tensions. Also the start up voltages for the servo can be an issue if the servos haven't been well cared for and these voltages should be checked.
Any competent shop with the right test gear should be able to check out the computer/controller relatively cheaply. The problem these days is less and less shops still have the test gear.
Once you have done the basics then you can make an informed decision on whether or not to repair or replace. In my opinion it's much better to spend a few dollars on some basic methodical troubleshooting than throwing buckets of money in an indiscriminate fashion at ad hoc repair attempts or rashly going out and buying a new autopilot.