NASO is $79/year and you get $3M worth of liability insurance plus some legal fees if assaulted. There is an option game fee protection insurance. For your $79 you get referee magazine with the special insert, NASO Locker Room e-mailed once a month, plus other info.
ABUA is $35 for $2M in liability plus similar benefits, including accident insurance, game fee protection and they offer various training materials and other training opportunities.
Check the respective websites for more information.
ABUA
http://www.umpire.org/
NASO
http://www.naso.org
Ed H.