Your "Willing to Pay" price should cover some tips to the traveller on top of the cost price of the item. There's no restriction to the value of tips you should pay, as the agreed price between travellers and requesters are based on a mutual agreement. You are encouraged to give about $5 tips for small and easy to get items, while bulkier or difficult to get items should offer a higher tip. A good gauge would be to imagine being the traveller for this item, and think of what you'd be willing to accept to help get it!
If you are unsure of the cost price, you can put a temporary price and negotiate with the traveller later.