Well, while being a newbie to this site here's my take on monitors for mastering.
If you are doing your CD for commercial release, leave the mastering to a mastering lab. This type of work is very specialized and those that do it well usually do nothing else, that's why they are good at it.
You can get a good mastering lab to do a 10 song CD for about $1200-$1500 depending on how good you final mix is.
I always send mine to a mastering lab.... they know their stuff and you will be glad you did.
Hope this helps