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 the customer will provide.

Multiply this number by the number of new customers your new premium URL might attract, including "type-in" traffic and "keyword" search engine traffic.

For example, if each of your customers is worth $1,000 dollars 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.
So, if you can buy a domain for a few thousand dollars that is worth tens of thousands or more 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 site to attract this search engine and type-in traffic.
Regardless of how many additional customers this domain attracts to your company, it should hold its resale value and in all likelihood grow more valuable over time.