Have a look around this forum... I'd say that Avis First isn't worth paying ANYTHING extra for. You can go to avisfirst.com and see the benefits. Basic benefits are
-Free upgrade (1 class) that's very unreliable, especially with the new Select & Go Locations. Have a look at the S&G thread and you'll see most people are either not getting upgrades or getting poor upgrades.
-Free weekend days. These could be of use to you, but they have a lot of blackout dates, require Saturday night keep, cannot be combined, you still pay a lot of taxes at most locations, etc.
-"Exclusive" website/phone #. Nothing special here, in my opinion.

The value of everything is dependent upon the beholder. You can look at the "benefits" and see if it's worth paying for an extra day. Maybe you can get a cheap-o weekend rate somewhere and get your last day for <$20. Then if you get some free weekend coupons for next year that you can use; it might actually be of use to you.