The bad news, Andrea, is that your car is probably worth between $8000 and $11,000 and maybe even less as a trade-in. Certainly that figure will be less if you don’t fix the engine problem, so that’s your starting point for working out the mathematics of your situation.
The one thing you haven’t told me is whether the car is a V8 or a six-cylinder car. If it’s a V8, I reckon you’d be spot on the money with a lifter, rocker or valve-spring problem as these engines were notorious for these components wearing out in roughly the mileage you’ve noted (sometimes less). The good news is that it’s not a terribly involved or costly (relatively) fix.
If, however, the car is a V6, then you might be in a bit deeper. These engines were even more notorious for stretching their timing chains and that can lead to all sorts of engine noises as well as a check-engine light illuminating on the dashboard. This is a bigger, more expensive fix, too, and since a V6 Calais will be worth less in the first place, it might be better to simply replace the whole engine. There’s also the very strong possibility that the cost of repairs will simply not stack up against the value of the car.