You have a very nice model 99 commissioned on July 8, 1926. Looks like the handle has been replaced on the bentwood top, and the decals could be in better shape....but the important part is that the machine works VERY nicely!
As for what you should ask for it? It all depends on what the machine means to you and what it's worth to you...keeping in mind that if the machine has sentimental value, that particular value does not translate into retail value. (In other words, it may be worth more to you than to others.) I wouldn't pay more than about $35 for it...and that would be because of the decal damage, and because I don't have an emotional attachment to the machine.
You can go to ISMACS.net and look up the serial number here:
http://www.ismacs.net/singer_sewing_machine_company/serial-numbers/singer-sewing-machine-serial-number-database.html
Where did you get the machine?