I have successfully implemented a budget autopilot on my 1985 23' Sea Ray. This solution is for inboard motor with stern drive. The hardware list:
1. Tinypilot computer, $100
2. Pypilot motor controller, $80
3. Raymarine st4000 Mk2 wheel drive
Total cost was around $360. Items 1 and 2 can be purchased at https://pypilot.org/store/ . I bought the wheel drive on ebay used for $145. New wheel drives are around $500. The computer uses a raspberry pi and the motor controller is arduino based. This is not for the non-DIYer. You have to be comfortable with wiring things up, etc, as well as understanding PID control. This was an attractive solution for me as I was loath to drop +$1500 on a new setup and I like to tinker. I engage the autopilot while setting lines and when a fish is on if I have 2 or more in the boat. When I am solo, I generally leave it on from start of line setting until I have pulled lines. Works well with my only complaint being the wheel drive is noisy (it's used and needs to gone over which I will do in the off season).
If anyone else is interested, I'd be happy to talk you through it. Drop me a PM.