For me it depends on a few things:
Extension? If its a .co.uk, im more likely to reg it than if its an org.uk - Im NOW far more strict on myself when it comes to buying org.uk.
Related Keywords? For example, if the domain is "BurglarAlarms.co.uk", how many searches do terms such as "Burglar Alarm", "Intruder Alarm" and "Intruder Alarms" get as you may be able to rank for them as well.
Product value/commission rate? I have certain domains which are ticking away selling a few products now and again earning a couple of quid per sale. I also have a few sites which make a handful of sales per year - however when they do its high £xx commission. If there is no obvious affiliate program, are there any obvious end users?
AdWord Bidders? If there are no bidders, its probably not a converting keyword.
Also depends with what sort of return you are happy with. If I can spend £7 on a domain, an hour on putting a site together and a couple of hours on link building and then get a site which will make me a couple of quid per week, Im happy. After the two years it would have made around £200 (assuming £2 per week), which would be enough to cover the time I put into it. I can then either drop it, renew it or flip it. Others on here wouldnt be willing to do the work for the same return though.