For some people, PRN is the best choice. Those are people who have access to group health insurance rates elsewhere (e.g. through their husband's employment) -- and who don't need steady income.
1. Decent health insurance can cost a LOT more than $200 per month, especially if you are in any high risk groups, want maternity, coverage, etc.
2. Don't forget dental insurance, life insurance, accidental death and dismemberment insurance, and disability insurance. You probably won't get ANY of those as a PRN -- and they can worth a lot of money if you find yourself needing them.
3. Retirment. Most PRN's get no retirement benefit. Many employers pay extra into your retirement plan if you are a full time or part employee. Mine pays an extra 4% of my salary into my retirement account. Over time, that money really adds up. When you are in your 60's, you are going to want a big retirement account.
4. Paid vacation and/or sick time. Calculate how much money that is worth. It can be a whole lot.
5. As others have said, if your unit's census goes down, the PRN's will probably be the first people to told to stay home. Can you afford to go several weeks at a time only working an ocassional shift?
6. If your employer decides to fill the vacant positions with full time or part time regular employees, you'll find your work hours greatly reduced.
As I said, for some people, the schedule flexibility of PRN is worth the risk of losing all of the benefits and the lower income that comes when you are not needed to work. But I have known a lot of people who have switched to PRN who later switch back to regular employment once they have some health problems and/or the census goes down -- and they have to go a few weeks or MONTHS without much of a paycheck.