Street sign cost vs Govt Mandated GPS?

Looking at all the very large Interstate Signs (“I-10 to New Orleans”) and even the smaller local street signs (name of the street), I looked up the cost of these signs. Here is what I found:

Sign panels:
* Regulatory/Warning/Marker: $15 to 18 / sq.ft.
* Large Guide Signs: $20 to 25 / sq.ft.
* Electronic Variable Message Sign: $50,000 to $150,000 each

Sign Posts:
* U-Channel: $125 to $200 each
* Square Tube (Telespar): $10 to $15 per foot
* Large Steel Breakaway Posts: $15 to $30 per foot
* Cantilever Sign: $15,000 to $20,000 each
* Sign Bridge: $30,000 to $60,000 each

Foundations:
* Square Tube: $150 - $250 each
* Breakaway Post: $250 to $750 each
* Cantilever / Bridge: $6,000 - $7,000 each

These are just raw material costs and excludes labor.

Source

Given theses costs, would it not be cheaper that the Government (us) mandate that all automobiles come with GPS within the next x years? I am not talking about Stop or Yield signs, just signs that are directional.

Portable GPS systems start around $90 but am sure the units that are integrated into your car are significantly more but holy crap, the cost of these street signs are incredible.

Am I missing something in my logic?