My personal favorite is some hyundai tiburon seats I picked up for $50. If you drive the car a lot a good seat makes a big difference.
$10 cup holder is right there with it
.5” taller ball joints make a big difference at $50 each if I recall
I also can vouch for the 97-00 Monte keyless system. Mine is tucked up above the door lock relays and works awesome for $30. My only gripe is the truck button is too touchy. I pop it by accident with my keys in my pocket.
Astrovan steering shaft is a steal at $15 from the junkyard
95-05 crown Vic transmission crossmember is great if your wanting dual exhaust. $20 at the junk yard
And $70 in cheap speakers because all of mine where blown when I got the car.
Any of these things can be done in a weekend and will be very rewarding after.