I’ve always had a problem with the word Priceless. I don’t feel it should be the word to describe something of extraordinary value, but rather should be a synonym for worthless.
Take the word tasteless, for example. When you say that a joke is tasteless, you’re saying it’s bad. You’re not saying that it’s so funny that it’s beyond even quantifying with a value of humor. Every time you use the suffix -less, it means it lacks something, and does so in a bad way, not in a phenomenally incredible way. Try calling your girlfriend attractiveless and see how she takes it, for example.
The word expensive should really replace the word priceless. Or just tell them it’s not for sale and quit being so pompous about it.