id   category cost currentdate

   1     type1   2400    2014-11-02
   1     type1   2400    2014-12-02
   1     type1   2100    2014-12-13
   1     type1   3000    2015-01-02
   1     type1   5000    2015-01-11
   1     type1   2000    2015-02-07
   1     type1   5500    2015-03-05
   1     type2   2700    2015-03-02
   1     type2   2000    2015-03-22

avg_cost   month  category
  nov-2014   2400   type1
  dec-2014   2250   type1
  jan-2015   2400   type1
  feb-2015   2000   type1
  mar-2015   2400   type1
  mar-2015   2350   type2

avg_cost   month  category
   nov-2014   2400   type1
   nov-2014   null   type2
   dec-2014   2250   type1
   dec-2014   null   type2
   jan-2015   2400   type1
   jan-2015   null   type2
   feb-2015   2000   type1
   feb-2015   null   type2
   mar-2015   2400   type1
   mar-2015   2350   type2