Is it cost-effective to purchase an expensive card?
For $2800 (the price of Monarch 500GH/s, as of 23 Dec 2013), and $4680 (the price of Monarch 600GH/s, same date), you might as well purchase a 1TH/s mining hardware, such as from VMC, or CoinTerra.

The other alternative is to use these mining hardware to mine some other altcoin, instead of Bitcoin, and hope that the other altcoin would come to prominence one day, instead of bitcoin. 

The answer to your question is that unless the difficulty drops, it is no longer cost effective to purchase mining hardware.