.uk.net?
New one to me and I've been doing this since the 90s...
I don't see the difference in this "domain" and using Biodiesel as a subdomain on any short tld to be honest.
I'd say reg fee, but I'm not sure what the reg fee is on a .uk.net, so I'm going for £0, unless you develop it & it becomes a success. If this does happen though, then I'm assuming that whoever owns uk.net can then charge you whatever they like as a renewal? Or maybe not.. I'm guessing here...