Contributory Parents Visa - Cancer???

Hi guys - hoping someone can be of help to me?!

My parents are wondering what the situation is if they wish to get a parental contributory visa when my Dad has had cancer? He's been clear for about 2 years (which isn't really that long), but has anyone had any experience of this, or similar?

Would they have to pay more?

Any information would be great - theres no point in them "umming&erring" over if its an option, if financially they'd never be able to do it!