Nz tax question

Hi All, not to sure if this is where this belongs. Started a small sole trader business in August last year and now looking to add some assets. I have a tractor that I use around 70% of the time for business use. I have had this valued by a tractor dealership and they valued this at 25k I only paid 20k 2.5 years ago. What should I use as the current value considering the I paid 20k and the value should be lower now?