New van/trade in , tax etc

Hi , I'm a sole trader (paying 40% tax) and bought a van worth 24k plus vat (I'm also vat registered) , when I fist bought the vehicle I got 100% aia capital allowances on it. My question is , that it's just over 3 years old and was thinking of trading it in and getting offered 11 k for it and I was wondering what the vehicles actually cost me .and it's been in the pool/books for 4 financial years . I'm a tradesman and not an accountant and just wanted a little help on this ,

Many thanks in advance Kieron