I'm not sure it's a design category question as such, but i'll ask anyway.

What if i want to make a commercial game, and have a character\device\vehicle from a well-known media franchise as a cameo or easter egg ? Would it be possible to get such a thing for free, and if not - how much would such a request cost me ? Say...say i want to have a Battletech's MadCat standing in a hangar, or TF2's Heavy walking around in the background ?

I'd rather have someone tell me from experience, instead of guesses.

You'd really need a lawyer to advise you on this. From what I have seen in the past:

Games often will parody existing characters or other IP(not a true representation), and usually get away with it. Likely covered under Fair Use:

http://en.wikipedia.org/wiki/Fair_use#Fair_use_and_parody

This Fair use is an exception to copyright, and likely won't cover accurate representations.

EA ended up attempting to do so in Battlefield 3 using real world helicopters without licensing them. They were sued and used the defense that it was Fair Use and also covered under first amendment rights. EA eventually settled the lawsuit, so we don't exactly know what the outcome would have been.

A lawyer experienced in intellectual property could advise you better on whether they think your usage would be covered under fair use. But remember, even if your lawyer thinks it's ok, you may still have to defend this in court which you probably don't have the money for. In EA's case, they have the money for that, and they likely intended to set legal precedence in such cases which would have greatly impacted what game companies could and couldn't do moving forward.