So there is still a small group of guys who love the platform (myself included). That being said, getting magazines, aftermarket DBMs, or even gunsmiths to work on them is getting more scarce and expensive. Couple that with modern rifles where the actions take prefit barrels, and the lust for a good FN SPR diminishes quickly outside of that small circle.
With the old factory DBM (not TBM) and upgraded (to some) bolt handle, you're probably looking in the $1,400 range with a +/- of $300. Several years ago you could still pick them up very gently used for less than $1,400. For reference: I bought a used first gen with Leupold Mk4 for $1,600 in 2016, a NOS A2 in 2017 for $1,400, and a LNIB A1 in 2019 for $1,150. I believe though that the prices have gone up a tad since then.
If you want to sell it, I'd suggest getting the supporter account upgrade and then reference your for sale ad in the FN SPR Enthusiasts Extinct thread here. That way you'll get it in front of more people who are interested quickly.
*** Edited to add that since you are in CA, you might get more out of a local sale.