I have an antique Singer foot pedal sewing machine with oak cabinet. The model number on the machine is G9628489. Can you tell me what the value of the machine is?

They can go for up to $150. Usually.

An antique dealer most likely won't give you more than $10.
You have to find someone that really wants one to make any money

Here are a couple ideas for you. Take some nice digital pics and post them on flickr or somewhere similar and then Google quilter guilds in your general area. They likely have websites. See if they have a Contact Us link and then let them know you have a Singer treadle machine from 1922 for sale. THOSE are the people interested in these antique machines... assuming it is working or at least looks like it is in good shape. Another thing you can do is Google the quilt fabric shops in your area and visit them. Talk to the shop proprietor and show them the pictures. Be sure to get the pics including the closed cabinet as well as it being open and fully setup for sewing. Do a closeup on the serial number, and then print the page from this website that shows your serial number: http://www.singersewingmachine.com/manufacture2.htm

I am sure you'd get some interest. Some quilt shops would be interested in having one in the shop itself, and quilters would love it for the nostalgia. Google Singer treadle and do the same on eBay. You are likely to find some there that will give you an idea for a dollar amount. I might be interested myself, if you'd ship. I live in the Chicago area... and I'm one of those quilter folks. Here is the link that shows when Singer allotted serial number ranges to its factories... and your SN appears to be from 1922.
http://www.singersewingmachine.com/manufacture2.htm