There are a lot of factors that make a domain name really valuable.
Generic / Dictionary words command the highest premium.

But here is a list of 10 domain appraisal parameters which are really important :

10. Search Engine Friendly :
Would a search throw up the domain intrinsically.
For example – if the exact search term exists in the name.

9. Industry Strength :
Is this targeted to a niche or a mass market?
A niche market product might actually have a higher value than a mass market product.
But it basically depends on the prevailing PPC rates for that market.

8. Sales Value History :
Has this domain ever earned any revenues?
How long has it been online? Longer the duration of the registration, better it is.

7. Market Potential :
Can you make revenue by advertising this?
Would it generate enough revenues if exploited?

6. Linguistically Viable :
Can the domain be pronounced?
Is it easy on the ears?

5. Brand Recognition :
Can it be created for this domain?

4. Recall Value :
If you can’t remember it, forget it.

3. Length :
Shorter the better
Chew on this, there are NO 3 Letter .coms available.
All 3 letter word domains for .in, .org, .net, ,biz, info, .us,
in fact most TLDs, are also sold.

2. Extension :
.com, .net, .ca, .in, .co.uk, .co.in, .com.au, .tv, etc.
.com domains are undoubtedly the most expensive.
speciality extensions follow.
3 Letter .coms now go routinely for $50,000+

1. Current Revenue :
Earnings/Parking/Affiliate/Other
Nothing better than current income, though not if you are spending thousands to start with.

So is there a formula for this? Or an appraisal system that is perfect?
Short answer is NO.
There is no hard and fast rule, one man’s poison is another man’s drink.
A good way to gauge the inherent worth of a domain is to auction it or invite offers for it.

Holding on to certain types of domains actually increases their value,
others might just cause holes in your pocket.