I'd guess somewhere in the neighborhood of $120, though you might want to start higher and creep down.
(I'm basing this on reasoning, not data, haven't ever seen a board-only 30g for sale):
With the recent Massdrop buy, the bar for fullsize Topre boards has been set at ~$200. If you pulled the keys off of one of those, the board would probably be worth ~$90. It isn't easy to get a 104 key Topre set these days, so it's a tough sell. I figure the 30gs are worth a small premium since they are much less common, but not a huge premium, as 30g domes aren't really a hot item despite their scarcity. I'd say to start around $160 and work down... maybe you'll get lucky and find the right buyer, but I'm kind of guessing lower $100s will take it.
Of course, others are welcome to chime in.