Hey Rob, Thanks for your comments. The prices are in Tongan dollars/pa'anga.
The duty free shop at the airport has a website - Leiola.com so that can give you an idea of what they stock. If you're after something specific it would be worth emailing them to check that they have it and what the price is incase the website isn't up-to-date (the website also says you can order online and pick up when you arrive). New Zealand has a much bigger duty free selection and I see you can look online for products too so you can compare prices if you want to.
Happy travels, I hope you and your wife enjoy your time in Tonga as much as we always do.