Impossible to predict selling price, its a thin market. That said, and even though I bought a perfectly fine 900/10 Apo-Saphir for 101 Euro last fall, 900 mm process lenses typically bring more than $500, sometimes considerably more. But sometimes less. Look for completed auctions for Apo Ronars and Apo Nikkors on eBay. These days Google returns mainly irrelevant garbage.

Just put the thing up for sale on eBay and accept what you get.