Anyone selling subdomains that wants to have a proper chance should make sure his subdomains are lower priced than the average real domain. Only in those circumstances I would consider buying a subdomain.
Some existing examples of what is NOT done:
.co.ni = 60 GBP per year
.ie.nu = 60 euro per year
.nl.vg = 12 euro per year
.uk.com / .eu.com / .us.com = most of the places are 30 per year or above, still way too highly prices
This is an existing example of how it SHOULD be done:
.nl.nu = 1 euro set-up costs, no annual renewal fees
This is the sort of subdomain I would consider because the price is so low you cannot do harm with it. For a temporary site you feel like spending very little on, or for some DNS experimenting, this is quite good. (however, the .nl.nu support is terrible so it may be a bad example)
As soon as a subdomain is asking more money than most popular real extentions, then I wouldn't touch it with a long stick. Must admit I would be tempted to buy some .eu.com's as this is a good extention and many nice domains are available. But the pricing turns me off. It is almost double of the prices for a .eu domain, so why pay 100% more for a subdomain instead of buying a regular .eu domain?