ok after staring at my chart. I think it would be too easy to get to 9 stars on any mount
so I came up with a new chart. STill makes the first couple stars easy. makes the the middle stars a little tougher, and keeps the over all costs about the same

White Horse Black Horse leopard tiger bear rhino dragon
exp @250k per feed @500k per feed @750k per feed @1m per feed @1.25m per feed @1.5m per feed @2m per feed
1 star 300 750,000 1,500,000 2,250,000 3,000,000 3,750,000 4,500,000 6,000,000
2 star 900 2,250,000 4,500,000 6,750,000 9,000,000 11,250,000 13,500,000 18,000,000
3 star 2700 6,750,000 13,500,000 20,250,000 27,000,000 33,750,000 40,500,000 54,000,000
4 star 5400 13,500,000 27,000,000 40,500,000 54,000,000 67,500,000 81,000,000 108,000,000
5 star 10800 27,000,000 54,000,000 81,000,000 108,000,000 135,000,000 162,000,000 216,000,000
6 star 21600 54,000,000 108,000,000 162,000,000 216,000,000 270,000,000 324,000,000 432,000,000
7 star 43200 108,000,000 216,000,000 324,000,000 432,000,000 540,000,000 648,000,000 864,000,000
8 star 86400 216,000,000 432,000,000 648,000,000 864,000,000 1,080,000,000 1,296,000,000 1,728,000,000
9 star 172800 432,000,000 864,000,000 1,296,000,000 1,728,000,000 2,160,000,000 2,592,000,000 3,456,000,000
10 star 259200 648,000,000 1,296,000,000 1,944,000,000 2,592,000,000 3,240,000,000 3,888,000,000 5,184,000,000
11 star 388800 972,000,000 1,944,000,000 2,916,000,000 3,888,000,000 4,860,000,000 5,832,000,000 7,776,000,000
12 star 583200 1,458,000,000 2,916,000,000 4,374,000,000 5,832,000,000 7,290,000,000 8,748,000,000 11,664,000,000
13 star 1166400 2,916,000,000 5,832,000,000 8,748,000,000 11,664,000,000 14,580,000,000 17,496,000,000 23,328,000,000
14 star 2332800 5,832,000,000 11,664,000,000 17,496,000,000 23,328,000,000 29,160,000,000 34,992,000,000 46,656,000,000
15 star 4665600 11,664,000,000 23,328,000,000 34,992,000,000 46,656,000,000 58,320,000,000 69,984,000,000 93,312,000,000
total needed 9740100 24,350,250,000 48,700,500,000 73,050,750,000 97,401,000,000 121,751,250,000 146,101,500,000 194,802,000,000