I just revised the proposal for prices and maintainance.
Speed 
 Road Cost 
 Road Maint. 
 Bridge/Elev. Cost 
 Bridge/Elev. Maint. 
 Tunnel Cost 
 Tunnel Maint. 

30 km/h 
 15.00 Cr 
 1.30 Cr 
 430.00 Cr 
 18.00 Cr 
 
 

55 km/h 
 55.00 Cr 
 4.60 Cr 
 510.00 Cr 
 16.00 Cr 
 
 

40 km/h 
 80.00 Cr 
 2.20 Cr 
 560.00 Cr 
 13.00 Cr 
 1,000.00 Cr 
 24.00 Cr 

50 km/h 
 100.00 Cr 
 2.80 Cr 
 600.00 Cr 
 11.00 Cr 
 1,100.00 Cr 
 19.00 Cr 

70 km/h 
 140.00 Cr 
 3.90 Cr 
 680.00 Cr 
 12.00 Cr 
 1,200.00 Cr 
 21.00 Cr 

90 km/h 
 270.00 Cr 
 7.50 Cr 
 940.00 Cr 
 16.00 Cr 
 1,600.00 Cr 
 24.00 Cr 

110 km/h 
 440.00 Cr 
 12.00 Cr 
 1,300.00 Cr 
 21.00 Cr 
 2,100.00 Cr 
 29.00 Cr 

130 km/h 
 520.00 Cr 
 14.00 Cr 
 1,400.00 Cr 
 23.00 Cr 
 2,400.00 Cr 
 31.00 Cr 

200 km/h 
 800.00 Cr 
 22.00 Cr 
 2,000.00 Cr 
 31.00 Cr 
 
 

The values are slightly revised (some higher, some lower) and can be further tweaked.
What changed is mostly in the LOGIC behind, a bit more coherent (less coefficients).
Here are the formulas used:
Kc = [1..8]
This is to group roads by type, as proposed above.
1 is for 30 km/h dirt road
2 is for 55 km/h gravel road
4 is for 40 and 50 km/h city roads, and for 70 km/h intercity road
6 is for 90 km/h intercity road
8 is for 110, 130 and 200 km/h highways
Road_Cost = (Speed * Kc) / 2Bridge_Base = 400
Bridge_Cost = Bridge_Base + (2 * Road_Cost)Tunnel_Base = 800
Tunnel_Cost = Tunnel_Base + (3 * Road_Cost)Maintainance was completely rethought.
The basic idea is that in
x months the total maintainance paid will sum up to the construction cost.
In other words, after
x months of maintainance the infrastructure can be considered renewed.
We need to play with the number of months.
M1 = [12..36]
This is the number of months needed to renew the road surface
12 months for 30 km/h dirt road and 55 km/h gravel road: in one year the weather ruins the cheap infrastructure
36 months for all the other road types: stone or asphalt need less maintainance
M2 = [24..48]
This is the number of months needed to renew bridges and tunnels
24 months for 30 km/h timber bridge
36 months for 55 and 40 km/h stone bridges and 40 km/h tunnel
48 months for all the other bridges (concrete) and tunnels
Road_Maint = Road_Cost / M1Bridge_Maint = (Bridge_Base / M2) + Road_Maint Tunnel_Maint = (Tunnel_Base / M2) + Road_MaintSome newer type infrastructures have lower maintainance than older ones, making it convenient to replace them when newer are available.
Specially, 55 km/h gravel road is rather cheap, but quite expensive to maintain.
Here's a new timetable, with costs: it shows the cost gaps in time periods:
In attachment: the updated Excel sheet.
EDIT: Added a few brackets for readability