SandisdeeP wrote: 
Right now this is the method you use to max out your Rares (R):
1 x Level 1 R with 10 x Level 1 N = 1 x Level 7 R (1 R and 10 N used) (2400 Gold)
Then you get two of these to get a Level 1416 R (2 R and 20 N used) (4800 Gold + ?)
Then finally you fuse 5 of these together to get a Level 40 R (10 R and 100N used) (24,000 Gold + 5? + x )
* note the ? is the cost of combining 2 x Level 7 R together
* note the x is the cost of fusing the 5 x Level 14 Rs
Alternatively you could do the following:
1 x Level 1 R with 10 x Level 1 N = 1 x Level 7 R (1 R and 10 N used) (2400 Gold)
1 x Level 1 R with 1 x Level 7 Rare = 1 x Level 14 R (2 R and 10 N used) (2400 Gold + 1400 Gold = 3800 Gold)
Then finally fuse 5 of these together to get a Level 40 R (10 R and 50 N used (19,000 Gold+ x)
Point is:
Alternative method is cheaper overall by:
24,000 + 5?  19,000 + 20,000
24,000 + 5?  19,000 is the saving with fusion costs, and 20,000 is the value of 50 Ns when sold (at 400 each)
Let me know if you have the exact figures or whether I made any errors in calculations.

For Top Formula
? = 2000, x= 8900
For 2nd Formula
x=8900
I figure out the cost for Fusion and it is easy to calculate as well.
The formula is
Fusion cost = (Base LVx100)+Base Rarity rate + [(Mixer Lv+x100)+Mixer Rarity rate]*
*if you use more than 1 mixer, you have to put [] for each of them.
Rarity rate is
N=100
N+=200
R=300
R+=400
SR=500
(List below is the rate I guess since I have yet to fuse them)
SR+=600
UR=700
UR+=800
EX.
1. Base (Lv.1 R)+ [5x(Lv.15 R)]
Cost = (100+300)+ [5x(1500+300)] = 9400 gold
2. Base (LV.1 R) + Mighty Slime + Mighty protoslime
Cost =(100+300)+(100+300)+(100+100) = 1000 gold
3. Base (Lv.32 SR) +LV.40 R
Cost = (3200+500)+(4000+300)=8000
The Alternative way to calculate with this formula is
Cost = (Total Lv of base and mixer x100) + Rarity rate of each monster.