Sounds like a Model C Series A. The box magazine Savage 99s are significantly devalued relative to the rotary mags; figure about $100 less for a Model C over a Model A or DL (presuming both rifles of equal condition). The box magazine variants were the last made; probably made sometime in the 70s or early 80s. A serial number range would narrow it down, if you're of a mind to care.
You are correct in that 284 is a very desirable chambering and will bring the value up quite a bit; should more than offset the box magazine.
Savage 99 values peaked about a year or two ago; a Model F in 308 that went for $500 in 2004 is selling for $425 on a good day. Seems like the only ones still selling for more than $500 are the pre-war takedown variants.
Two years ago, a Model C Series A in 284 would have commanded $750, and may have gotten it. Today - maybe $600 but likely closer to $500. Dunno for sure, because (as you point out) they're just not that many out there to use as comps (and goodness knows I'm no expect on 'em).
