How much methane clathrates are buried in continental deposits?
According to Role of methane clathrates in past and future climates, 


  Methane occurrences and the organic carbon content of sediments are
  the bases used to estimate the amount of carbon currently stored as
  clathrates. The estimate of about 11,000 Gt of carbon for ocean
  sediments, and about 400 Gt for sediments under permafrost
  regions...


So 400 billion tons.