Self-studying the CCNA is remarkable accomplishment so you must be a disciplined learner. The CCNP is equally if not more difficult to study solo since the exams are increasingly more difficult and also contain those nasty simulations which will require you to be familiar with a wider range of equipment types and configurations. The Boson simulation software (I assume you are referring to their Netsim product) luckily covers a portion of the equipment and configurations, but will not ultimately guarantee certification success by itself since the exams grill you above and beyond.
As far as a salary, the average salary in 2004 for a Cisco Certified Network Professional was around $72.000. However, value of the certification goes beyond the salary, especially if you are in the market for a new job.
This was first published in February 2005