If your concertina has full-width hand-straps and each button plays a different note on push and pull strokes, it's an Anglo. This is the instrument of choice for Irish players.
The cheapest ones have just two rows of buttons and brass reeds. These have a very limited range, and will be worth only between £200 and £400.
30 and 32 -key models with steel reeds, such as the rosewood-ended Lachenal, will fetch £1800 to £2200.
Almost any Anglo bearing the name Crabb or Jeffries will be worth a minimum of £4000, and the very best of them could fetch half as much again. Bear in mind, though, that these values apply to fully restored instruments tuned to modern concert pitch. Unrestored instruments may be worth 50% less.