To me it is a pretty exotic collection topic. Is it difficult/expensive to collect?
Not difficult, Japan Revenues and seals have a HUGE array,
and I agree, they are handsome and intriguing stamps.
Price? this is relative to where you are and your disposable income.
I was picking them up for years at about 20c to 80c each
Now (Aussie Ebay the cheapest is around $7...outrageous!)
Americans have an embarrassment of riches to pick from,
prices and postage would be envious to me.
Because of price, I stopped collecting them 4 years ago.
Here is an Oz example $32 (c$20 US) =$3 each