MM was renowned for their attention to detail with even the props, so I doubt it was as cheap as one might think, Breaking Bad had some impressive visuals (not effects, granted, and due largely to the scenery which was in a cheaper-to-shoot state), but Walking Dead can't be cheap, nor the spin-off, which has had some pretty big sets, in addition to the zombie effects. We're not talking zombies overrunning an isolated farmhouse here, but beaches, city riots, army bases.
Mad Men had a budget of around 2 to 2.5 millions per episode. That's pretty lavish for a show without much VFX (still a lot of digital painting in exterior scenes to de modernize the city), but that owed a lot to the fact it's a period piece with maniacal attention to detail, and with a big cast, some of whom got pricey.
Walking Dead started at around 3.5 millions but it got cut since to around 3 millions.
For comparisons's sake, a VFX-heavy show (pretty lavish) like The Expanse runs at around 5 millions per episode for its first season, and this is on SyFy, hardly reputed for its deep pockets. It's only an American show because the production house in from LA, though. It was entirely produced and post produced in Canada, when the dollar was worth 0.65 US$, so they really got a lot for their money. But then, GoT does much the same, filming where it's much cheaper than in the US, and having a lot of the VFX produced in Canada or Europe.
Netflix is a bit discreet about the cost of their shows, but for the Expanse they got outbid by Syfy (so was HBO). It still gives an idea of what they might be willing to pay per episode for niche content like SF or Fantasy shows adapted from popular franchises.
Bottomline: the budget gap between HBO shows and other shows is decreasing, because more and more content-providers intend to grab a piece of HBO's pie. The Expanse on Syfy is more expensive than Boardwalk Empire, Deadwood and the big networks' shows like Fringe or Lost. And GOT, IRRC, started with a budget of 5/6 millions, equivalent to what Syfy is now putting on a show...
I don't think budget would be a major problem for a WOT show, especially not for its initial season, and even less so if they do like a lot of major shows and avoid casting, filming, post producing in the US where costs are prohibitive (people forget that Friends used to cost as much as GOT...).
To be honest, I don't think WOT is ever gonna get a GOT-like budget. It just doesn't have what made GOT break into mainstream, so it might as well not even try and play to its strengths instead. It's a good news in a way, because it means it really has to be successful as a niche show, and much like The Expanse has done, make very little concessions to mainstream viewers, focusing instead on being attractive to book fans and genre fans.