I rarely buy new registrations. From my perspective, you lose £3-5 per self registration for every one you let drop (obviously).
I tend to buy up smaller dropping domains on a larger basis, let them pay for themselves (if they have viable metrics) with genuine unique content, until there is a potential sale. This way, you never lose, because if you cant earn £3 within a year via advertising on a viable dropped domain, your doing something wrong. Let them pay for themselves and be self sufficient until you can;
1 - Build the domain and increase its value
2 - Luckily drop on a brand in which hasnt been created yet (icloud)
3 - Sell it for 1000's % Profit
One sale out of 10-50 purchases can put you back in the black!