But to add a few rough figures to this I have 3 minisites with similar domain quality to those listed (so pretty average you might think) but they are each bringing in > $150/month in PPC+affiliate revenue
Each of the sites were built in 1 day and it took another day to do some seo and get backlinks.
Now I'm not saying that all attempts to do this will be successful. I've tried and had very poor results with other names. But if someone thinks that a domain available for lease can offer a higher PPC than if they used a free to reg name then £10/month is cheap isn't it? If depending on their skills/domain they can make $150+ like my example above...
Well thats the beauty of working on your own sites. If you built 260 of them in a year (one per work day) and only 10% of them made $150/month, the following year you're on $4k a month without any further work. Not long in quickly building up a nice passive income.
The thing is with most of your domains I honestly don't think there is any advantage to building a site on them over a freshly registered domain. If it was £10/month its not even the price thats the deciding factor - its the fact you now don't own your own income stream. There are already enough factors that could get my passive income taken from me (google algorithm changes, link buying penalties, interlinking penalties, etc etc) without adding another one of a person just deciding they dont want to work with me any more and going out on their own.
Food for thought. I don't know anyone who has a name leased but I think that if this does happen it probably is with much bigger names like your example of creditcards.co.uk - but that certainly will be a lot more that £50/month..
We've got some big domains leased, though joint ventures would probably be a better description. The sites are doing low four figures a month in profit, the domain itself would probably be worth £6000-7000 a month were it not for the profitable site already on it. My company done 100% of the work on it - the domain owner does nothing but bank his money. It suits us both though as he's already made more than if he'd sold the domain outright, and now owns half of a website that would probably sell for low 5 figures as well. It suited me for two reasons, the first was that at time I couldn't have bought a £6000 domain because I didn't have £6000

. The 2nd was that in order to rank and make that money, an exact match domain was required. Since there is only a .com, .net and .org (.co.uk in uk too) that have any value whatsoever, partnerships like these are the only way to even get in the game at times.
I know you didn't think much of my domain list but take a closer look there are some nice names on there. A few which are XXXX+ names: (liam.co.uk, studentbooks.com, londonstudent.co.uk, morecambe.co.uk...) They aren't all that bad
Liam.co.uk is a great domain if you're called Liam. It has no value purely from an seo point of view - people searching for "liam" can't be monetized. If you gave me pick of those domains, the only one I see value in for developing would be the Morecambe one. I'm surprised how little search volume there was for "student books" though - I'd have thought that was your best domain from that list. For the right buyer Morecambe might be a xxxx domain, I've no idea on the size of that town so wouldn't like to guess. If I was called Liam (I'm not) I'd pay £1k just to have that for an email address/personal site. Studentbooks and londonstudent are xxx, imo. Of course we can both say what something is worth, but ultimately its worth what someone else is willing to pay for it at that moment in time.