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.