How bad is it to use expired neb treatments? I have severe asthma but no money to see a doctor so that's my only option.

Some other thoughts. This is obviously a difficult problem. If one has severe asthma and it is not treated properly then there is the very real risk of serious consequences and costs (from being in the hospital).For financial hardship cases, it might be possible to work out a payment plan with a local doc. Most asthma docs would much prefer to tx pt and work out the finances then see one in the hospital or worse.
Depend on age of med. Of course, it is best not to use expired medicine if you do not have to. The danger of doing so is that the medication may lose some of its effectiveness over time. So you may be giving yourself a treatment, but may not get any relief from it. It just might not work. That being said, if an expired medication is not too old, in all honesty, it probably will help. If it doesnt, go to an er!
Patient assistance. If the nebs are not too expired, it is likely safe to use them, and they will probably work fine, espeicially if they have been maintained in a clean, cool environment. Most meds lose potency when they expire but do not become dangerous. Pills are often stable for many years; liquids less so. Your doctor or county medical society should know about patient assistance programs for meds for you.
No magic number. The expiration date on a drug generally denotes a loss of 10% activity. However, some drugs' preservatives may give out leaving a potentially "contaminated" drug. Always best to check with your pharmacist about specific drugs. Some drug companies ddo have assistance programs. In your case those who make asthma meds should be checked out (there are a number of companies). Best of luck.