Nobody knows what the difficulty is going to be, so lets make a table of possible values for a 1 Mh/s PMB at the current difficulty.

Expected Increase	| Total Dividends
30%	0.00157759
20%	0.00218436
10%	0.00400466
7%	0.00556492
2% (Moore's Law)	0.01856706
0%	infinite

EskimoBob and others are wrong. You can see that it is possible to make money from a PMB if you buy it at the right price. So, figure out what you think the difficulty will rise by every 2 weeks over the next several months, and don't pay more than the dividends you will receive.