Ebay sells 3rd party chargers at just US$8. I've been using them for quite a while with no problems, but use it at your own risk. They are also not as durable, but should be able to last a year under heavy usage (use and throw type). The original chargers are just too dang expensive, 1 original charger = 6 3rd party chargers.
One thing to note is that 12V/2.58A chargers work only with SP4 and (maybe) SP5. Doesn't work with SP6 - I guess SP6 has better power protection capabilities to prevent it from working with those cheap 3rd party chargers.