The monthly charge for the 1st Hopper is for DVR service, for ALL receivers on the account, regardless if they are owned or leased; it's not a "receiver fee", as the 1st receiver is normally included as part of your service.
As far as the different prices you stated, $15 is the current "rack rate" for DVR service with Hopper receivers, while $10 is when you are in some sort of promo pricing, which I am in & is what I'm currently paying.
However, my monthly bill does show my DVR service at a "loyalty price" of $12, then a $2 promo credit, which brings it down to $10.
Then, my 2 other HWS receivers also show a monthly price of $15, but are then only billed at $12, again loyalty pricing. (all my receivers are leased)