The traffic signs were not stand alone pieces. They were part of a package of elements that also included things like the Welcome signs, Transportation and Ticket Center colors and signs, and Magic Kingdom toll booths which have also been changed. The traffic signs are a part of a larger project.
This actually makes me feel a little better about the road signs because as a whole I like the new color scheme. I think it works better on some things (welcome arches, MK sign, etc), worse on other things (road signs, etc), and I'm kind of indifferent on others (TTC signage, monorail signage)... I'm glad they're keeping a unified theme across the whole resort rather than random colors everywhere though.
I’m not sure why every unpopular decision Disney makes is branded as cheap. Aren’t people aware of the enormous sums the company has spent on its recent projects? Whether the money has been well spent or not is another question, but cheapness isn’t the issue here.
In this case I totally agree, there could be a slight variation in price but I doubt it factored into the color palette decisions at all. Paint costs more or less the same whether it purple or blue or yellow or red, likely true of reflective sign coatings, etc.
There's hundreds of areas where Disney is penny pinching but I don't think this is one of them. I think they just decided the old colors were dated and picked a new color palette.