In metal working circles those stamps are known as touchmarks. $120-$140 is very reasonable. Here is but one more link to another firm which makes quality touchmarks. I made the few I use on my own ironwork, very tedious yet simple. I prefer to spend my time at the anvil and not the file bench if I can avoid it. If I need replacements I'll farm the work out because folks who concentrate on this sort of work can produce a better finished product than I can.
http://www.incandescent-iron.com/