Are they same to say "$100 only" and to say "only $100"?
Good question! No, it would not be the same to say "only $100" in this case. This is because "only" in the original sentence is describing "a year ago". The "only" in "only a year ago" emphasizes that it's been just one year, since these items costed $100, which isn't very long ago. If you were to instead say "only $100", the "only" would change the emphasis to the $100, and it would mean that these items costed only $100 a year ago, which would emphasize that it wasn't that expensive. The word "only" can change the entire meaning of a sentence based on where it is placed, so it's good to be aware of this.