For less severe weather conditions, effective covers from FotoSharp work quite well. They are a simple design ans a lot cheaper. As well they are available in camo. So you don't have to be advertising for ThinkTank, Optech or Aquatec.
And they're light weight enough and pack small enough to take just as a quick cover if a shower comes over the ridge. Priced right at ~$45 for a camo cover for the 500 f4.
Not sure if you get these in the US but I have used one of these (http://www.wildlifewatchingsupplies.co.uk/) on my 500 prime for years. Easy to get on and off in a hurry and cheap!
If I have real need of protection, I'd agree with the TT Hydrophobia. However, for cheap and cheerful I stick a few Op/Tech Rainsleeves in my bag. They do a pretty good job.