In the U.K. "What's the time?' is widely used as well as "What time is it?" and "Have you got the time?"
The above comment really made me smile! What I like about the Finnish course on Duolingo is this quirky teaching method of giving bizzare example sentences e.g. 'Tämä veitsi haluaa käydä Sveitsissä.' and also using rather literal translations, among other Duolingo's unique features.
Yeah, at times some of the translations, sentences etc. may seem unnatural, odd or incorrect. But I usually turn to the comment section if I stumble upon such an occasion, which in most cases will give me some answers, otherwise I look up elsewhere for more info, especially grammatical rules.
You are right that literally taken the questions are confusing:
- Paljonko kello on? : (lit.) How much is the clock?
- Mitä kello on? : (lit.) What is the clock?
It is just the word aika : time is not used when inquiring what the time is. It is used when you ask how much time you have remaining:
- Paljonko on aikaa junan tuloon? : How much time is there before the train arrives?
If you ask the price of a clock, you use the verb maksaa , to cost, to have price, or ask hinta, price.
- Paljonko tämä/tuo/se kello maksaa?
- Mitä tämä/tuo/se kello maksaa?
- Mikä on tämän/tuon/sen kellon hinta?
Note here the three dimensional demonstrative pronoun system (tämä/tuo/se) comes handy to make a difference between objects.