As for Viper, main issue for me not it's price. It's pretty inconvenient payment model. To pay you have to use PayPal, so it's a multi-step task:
- activate necessary effect, select PayPal, wait about 5 seconds
- scan appeared QR code with smartphone (yep, you'll need a smartphone for that) and go to PayPal site
- enter password there (luckily I have 1Password, but without it, it would be a pain in the …)
- repeat for each separate effect
And now, surprise. I'm using Fiio X7 too, so, I'm not only having to pay twice (and may be more, if new players will come out), but I have to repeat those annoying steps more and more times.
Actually, Viper effects are really good, and I like some of them with particular tracks, but I'd luckily pay $10 more for DAP, but avoid all that ridiculous movements.
And definitely I'd like to activate them for all my devices via single "lifetime" purchase, as it usually done for mobile apps.