I don't know why Dish doesn't let you add it to any package. Seems like they are losing money by not doing so. I'm not going to pay an extra $15 a month just to get it. It comes with any package from my local cable company- Xfinity. If I wanted it I'd go with them- cheaper.
Xfinity in my area doesn't offer a package cheaper than AT120+ with RSNs in it, at least not as a regular rate. Last I checked, AT120+ was $50 and the cable company's lowest package with RSNs was $58. Of course, the cable company also included NBCSportsNetwork and MSNBC (Which aren't in AT120+) in that tier. So, one might consider cable the better value overall in that comparison (i.e. getting channels you can't get with Dish in a similar package, for only a few extra bucks instead of $10 or $20 extra).
But at least where I live, it isn't true that cable offers RSNs in like a lifeline type service or anything. It's $58 regular rate minimum, after the lower promotional rate for the first or first and second year. And in my area the cable company refuses to carry one of the overflow channels for the RSN that covers the local hockey and basketball team- so if they both play at the same time and the RSN has both games, on cable I'd only get one of the two rather than being able to choose either one on Dish.
Anyhow, not being an apologist for Dish here by any means. I've been with cable before, I may be again. There's stuff to like and stuff to dislike about both companies. And I'm with you that I'd love to see RSNs in a lower tier package, or at least get NBCSportsNetwork and MSNBC for the $50 AT120+. It's ridiculous that to get NBCSportsNetwork and MSNBC you've got to go to AT250, which costs way more than cable.
But just wanted to be fair and point out that at least in my area the lowest cable package with RSNs costs more than Dish's lowest package with RSNs. Might be different in your area, though.