Once you use /rankup, there is not an easy way [that I have found 
] to see how much previous ranks cost. Below is the full list:
A to B: $200,000
B to C: $550,000
C to D: $1,946,880
D to E: $2,336,256
E to F: $2,803,507
F to G: $3,364,208
G to H: $4,037,049
H to I: $4,844,459
I to J: $5,813,351
J to K: $6,976,021
K to L: $8,371,225
L to M: $10,045,470
M to N: $12,054,564
N to O: $14,465,476
O to P: $17,358,572
P to Q: $20,830,286
Q to R: $24,996,343
R to S: $29,995,611
S to T: $35,994,733
T to U: $43,193,680
U to V: $51,832,416
V to W: $62,198,899
W to X: $74,638,679
X to Y: $89,566,414
Y to Z: $107,479,697
Total A to Z: $635,893,796
] to see how much previous ranks cost. Below is the full list:A to B: $200,000
B to C: $550,000
C to D: $1,946,880
D to E: $2,336,256
E to F: $2,803,507
F to G: $3,364,208
G to H: $4,037,049
H to I: $4,844,459
I to J: $5,813,351
J to K: $6,976,021
K to L: $8,371,225
L to M: $10,045,470
M to N: $12,054,564
N to O: $14,465,476
O to P: $17,358,572
P to Q: $20,830,286
Q to R: $24,996,343
R to S: $29,995,611
S to T: $35,994,733
T to U: $43,193,680
U to V: $51,832,416
V to W: $62,198,899
W to X: $74,638,679
X to Y: $89,566,414
Y to Z: $107,479,697
Total A to Z: $635,893,796