About how much would replacing a septic tank cost me?
I found some rough estimates at costhelper.com


  Installing/replacing a conventional
  septic system (including the tank)
  averages  $2,000 -$5,000 in the
  Midwest, but can be  $4,000 -$12,000
  or more in areas where materials and
  labor rates are higher.
  
  Enhanced, engineered or alternative
  septic systems that use mounds,
  sand/peat filters, aerobic systems
  and/or constructed wetlands can cost
  $10,000 -$20,000 or more, according to
  the Rhode Island Regional Water
  Quality Program. These alternative
  septic systems work better than the
  conventional approach for sites with
  high groundwater or slowly/rapidly
  percolating soil, or near drinking
  water supplies, wetlands, coastal
  ponds or other water resources.
  
  The cost of a septic tank alone runs
  about $500 -$1,800 depending on size
  (ranging from 300-1,000 gallons) and
  type. Piping and other needed items
  adds another $100 -$200 to the total
  cost of materials.