Purchasing a Domain from a Domain Squatter
If you're visiting the domain you want to buy, and it's hosting ads. IMMEDIATELY STOP VISITING IT. Domain squatters are able to determine value based on traffic. If the site is getting zero traffic and you come in and say you want to build a website about your pet cat on that domain, odds are good they'd be willing to sell it cheap to recoup the registration fee they paid for it.
However if they are getting even a few clicks on the ads, showing regular traffic from you visiting it, and you make the mistake of saying you're going to build some huge awesome website that will make you millions, good luck getting it for anything less than $500.
Keep in mind, the squatter is always willing to wait longer than you are for the deal to go through. Also odds are high about you being ripped off if not using a 3rd party to verify the transfer.
If you're the unfortunately soul that purchased your domain through goDaddy and noticed your .com version is being squatted, odds are it's the goDaddy squatter team and you're in for a very very expensive situation to get it from them. I would just give up and look for a new domain unless you have some deep pockets.
The price would depend on the greed of the domain owner and perceived value of the domain. I've seen instances in the past where domain squatters have demanded anything from £1,000 to £10,000 for a .co.uk domain (they cost £6 for two years). Needless to say we have found alternative domains.
If you can prove a trademark on company name/product/brand it does change the game a little since you can legally take back a domain name that infringes on a trademark. However in practical terms it would cost a lot of time and money to release a domain that you felt was being held unethically by using your company name/branding in order to sell. If you care to read more search for "trademarks and domain names".
Practical advice: it is likely the person holding the domain will demand an unrealistic price initially to test the water. Have a maximum price in mind before hand and never go over this value. Wait for the initial price demand from the domain owner and if it is in excess of your figure (almost certain to be), make your offer and wait a while. If they have no other offers on the table their greed may get the better of them and they may come back and accept the offer. Otherwise, get another domain name.
It would be great to see domain squatters stopped, but unfortunately this is unlikely to happen any time soon.