Sounds like a mid-20th century machine, and it would fetch $0-50 around here. Test it to make sure it sews well before offering anything. I paid $10 for a 1954 Singer 15 (a good machine!) and actually overpaid for it because it was one of the Centennary of Sewing badged Singers.

If this is a treadle, rather than electric, most of them around here are going for $25-100. Again, make sure it sews, and make sure you can get needles for it if you want to actually use it for sewing.

First find out how much your mom is willing to shell out and then offer the seller a little less than what you were given by your mom.Then if they take the bait you just made a few buck or you could go and give the change back to your mom.

just because its old it does not mean that its an antique ask how much you may be surprised with the price

Offer them 25.00. That is all it is worth