Reasearch I've done about expired asthma meds show that while they may lose potency over time, they are not unsafe to use.  I personally have never used an expired Advair inhaler, but I have used an expired Albuterol inhaler with good results (although the med starts to taste funny after a while).  I'm not encouraging you to use expired meds, just throwing that bit of information out there.

There may be some options available for you.  I will provide you some links that might help:

I don't have health insurance and I can't afford advair, What can I do?

affordable medications

You might also want to discuss this with your physician, as perhaps he might be able to prescribe an alternate medicine to control your asthma that is less expensive than Advair. 

Good luck.