Most of these machines are only worth a couple of hundred dollars but the conversion might lower the value on this unless the treadle was kept intact.
Condition is key. Wood in good shape, working, etc.
You can call Singer with the serial number and they will tell you when it was made but don't even ask for a value. I have a mini guide for dating these machines based on serial number on my website - http://www.mygrannysatticantiques.com/html/dating_singer_sewing_machines.html