ENDP's buy range for small cap companies is $1m-$500m. Nasdaq lists ZGNX's market cap at $596.75m which puts it out of ENDP's price range. ENDP is overleveraged with all their recent acquisitions while MNK has lots of cash on hand. A $1billion valuation of ZGNX(assuming Zohydro will be a blockbuster) is worth about $7 per share; add Sumavel Dosepro revenues and your looking at close to $9. A multi-billion dollar valuation puts it sky high. I think MNK can afford that.
If ENDP was to make another acquisition it would probably be BDSI since they currently have a world-wide licensing agreement for BEMA Buprenorphine for pain. BDSI's current market cap is about $236.63m which is in ENDP's price range. A $500m valuation of BDSI (ENDP's high range for small caps) is worth about $13 per share. BDSI has a PDUFA scheduled in June'14 for Bunavail, and phase 3 results for BEMA Buprenorphine due 1Q and 2Q 2014.