Depends. Until you tear into it, could be just a hose connection, hose, or the coil ends, which still means the whole coil. It looks like AZ wants $55-95 for one. Plenty of U-tubes to take the job from a difficult time-consuming trauma, - to just a simple time-consuming nuisance.
But you got to admit this is much more interesting than just changing a bunch of interior bulbs to LED right? (other thread lol) I mentioned before you traded up and scored nice v. your prior Marauder. You just might want to eyeball any number of things that could actually be in worse condition from "too few" miles as opposed to the opposite (seals, gaskets, hoses, belts....)
I know you already replaced the heater core but FWIW I had the same issue and used k-seal to fix it. No leaks in over 6 months. Eventually I will replace my heater core but for a quick fix k-seal is great and supposedly is a permanent fix. A friend of mine had a similar issue in his 4-runner and successfully used k-seal tot fix his leak too. I have since drained most of my coolant (minus the block) when changing my opti and water pump and still no leak in the heater core after 2000 miles on the new opti. I'm sure someone will have a contrary experience to mine but I just wanted to share my experience.