Here is a website showing how it is done.
Apparently, you can just lock the seat into place without pinning it or welding it, if it is a stock seat. I have never replaced a stock seat. Every time I have replaced a seat we were always moving up in valve size. We also welded them in place and then machined the seat back to size. I guess with a stock seat this doesn't need to be done; therefore, it should be really cheap compared to the price you were quoted.