Is this the kind of thing tracked at https://speedmaster101.com/ or https://www.fratellowatches.com/speedmaster-price-guide/ ?
Otherwise this might help: https://watchcharts.com/watch/119/omega-speedmaster-moonwatch-311-30-42-30-01-005
Recently they’ve been getting snatched up quickly (sold within days) when listed sub-$4300. You can still buy them new from Jomashop for $4995. You can also comb watchrecon.com daily and find them as they get listed.