Just my 2 cents:
Unless you're planning on making a business of constantly creating new sites or KNOW you can create a new one that will generate as much steady income or more than the old one (which is unlikely since it's hard to tell what a site will EARN before it's built) BEFORE the n months time during which you use up the money you can make from the old one at the current rate, then there is no sense in selling at all.
Compare it to "flipping" in the real-estate biz. If you don't get it rebuilt, within the number of months you plan on (limited by how much you sell for) and ON budget labor and materials-wise, YOU LOSE MONEY overall on the deal.
You have to consider what YOU COULD be making from it YOURSELF over the next n months just for letting it SIT there or with minimal maintenance work vs. having to develop and labor during that same period of time to regain that level of income. You have to consider how long months-wise (without the additional Adsense income) and how much physical time (hourly wage you want to make) of your own or hired work it's going to cost you to create a new one. Is your time free? If you're not at LEAST going to break even in n months time, there's no sense selling.
On the other hand if you're simply tired of the old site and want to get out of the internet rat-race, or are afraid Adsense is on it's way down the drain... Then by all means sell, but keep in mind the intrinsic value of the domain name too, and enjoy your retirement and lump sum of money.
Frankly, if it was me, I would do like someone alluded, and what just about any other company who is planning on selling soon does, work on the bottom line and optimizing the pants out of Adsense and Google/Y SERPs to get the monthly income as high as possible. Like they said a 1000 page site, with good traffic, should be doing 5-10x better than $20/day.