To estimate the value of a Domain, consider what the value of one new customer is to your business, including its lifetime residual value and all of the referrals that customer will provide. Multiply this number by the number of new customers your new premium URL might attract, including "type-in" and "keyword" Search Engine traffic.
For example, if each of your customers is worth £1,000 over their lifetime, and your new URL attracts 50 new customers over the life of your company, then the Domain could be valued at £50,000 to your business. If your premium URL attracts 10,000 new customers and they are valued at only £5 each over their lifetime then you would come up with the same £50,000 valuation.
It follows that, if you can buy a Domain for a few thousand pounds and it is worth tens of thousands to your business, then it's clearly a bargain. In theory, the more premium Domains you have that generate site traffic and new customers, the better. All premium Domains can easily point to your existing Website pages to attract the Search Engine and type-in traffic, although there is no need to overdo it.
Regardless of how many additional customers a Domain attracts to your company, it should keep its resale value and in all likelihood grow more valuable over time.