So I read from dualshockers and indiewire that the redesign of Sonic costed them around $5 million:
According to
IndieWire, the actual cost to redesign was closer to only $5 million. This information comes from a source who stated that most of the visual effects work in the film wasn’t complete in the first place before Paramount decided to delay the movie until 2020 to overhaul Sonic’s design. Seemingly, the only finalized work for Sonic on the film that had been completed was what was shown in the initial trailer for the movie. Because of this, the decision to change Sonic’s look was easier, and cheaper, than what many may have initially expected.