=DLookUp("[Cost per Metre]","MainTeesQ","[txtMainTcost] =" & [txtMainTcost])
This is wrong. the lookup function is used like this:
Dlookup(field1, table, field2 = value)
field1 - the field whose value you want to return
table - the name of the table/query the field belongs to
field2 - the name of a field whose value you know, e.g a ID field
value - the value of field2 that you know
So your code should be
me.txtMainTCost = DLookUp("[Cost Pre Metre]", "MainTeesQ", "[MianTeesQID] =" & IDValue)
This assumes that MinaTeesQID is the primary key from MainTeesQ and IDValue is the value of the record whoose [Cost Per Metre] you want.
Hope this clears things up.
ntp