Since only one SQL server is active in an Active\Passive cluster, you should not require multiple Monitor licenses.
P Curd commented
Mirroring is another area where you have two or three machines running on a single SQL license - I couldn't convince finance to let me buy three licences of monitoring to cover one license of SQL. I expect to have to pay more for it since it's doing more work, but I can't justify three times the price.
Thanks a lot for your feedback. We will definitely try to improve licensing.
Red Gate does offer some discount on cluster license. So if you are buying SQL Monitor license for cluster then please do mention this to Sales team and they can add some discount to your license purchase.
Nathan Bell commented
You should offer price points either by per instance or by per machine. This woudl grant users the ability to license either way for shops that run many instance on one server and vice-versa.