Us, UK, selling to a Distributor, NI, but shipping direct to a customer in NI - What to put as the value of goods?

When we started this, the idea was we would ship to the distributor, and that is us out of the way, what he does is his business, but somewhere along the line, someone decided we would ship direct to customer

Now on the paperwork to NI, we need to put the value of the goods in each shipment, however, we don't know what price the end user has been charged, just what we charge our distributor, and we don't want to put £500 if they are being charged £5,000, both to avoid any misunderstandings with customs, and to avoid upsetting customers who might feel they're being ripped off

Anyone have an easy solution?

I suppose we could ask the distributor for the pricing per order, but that seems a lot of faffing around?