I don't think it's a money problem. The series was created by Filoni, the president and chief creative officer of Lucasfilm. Plus we've had Vader speak in Rebels, so he's allowed to speak in animation. Plus getting Vader to speak at the moment is "free". His voice got thrown into Fortnight or whatever.
The idea here was definitely to be the force of nature Vader, similar to how we see him at the end of Rogue One. You can't talk to a force of nature and a force of nature doesn't talk to you.
I could see Vader having a larger role in future seasons, and he might talk then, but for Maul's first meeting against Vader in think this worked well.