The first no number/monster cover issue is incredibly hard to find.
Unusually, a CGC graded 7.0 copy, the Highest Graded so far, only fetched $47 in a Comic Link auction in Jan 2017.
The second one with the Ogden Whitney Effiel Tower cover turns up from time to time. You would think it would fetch big bucks but truth is the contents aren't very interesting. A VF Minus copy fetched a mere £7.99 on eBay UK in Dec 2014.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.