Does anyone know how much a dollar bill, with a blue stamp,( I believe it's called a silver certificate), is worth.
It's pretty worn but not torn or anything.

Most silver certificates are valued by their condition and date. A blue seal silver dollar in unused condition might fetch $10 on eBay, or more if an early date, and where printed, who signed it, etc. Look on eBay for the same year and condition, that should give you a ballpark figure.
ps It's also supply and demand, if everybody is selling them, then price will be lower.