From my knowledge, these are the price of fast trainers in different levels:
Level 1
3* = 900K-1.2M
4* = 1M-1.4M
5* = 1.1M-2M
Level 2
3* = 4* L1
4* = 5* L1
5* = 1.9M-3M
Level 3
3* = 4* L2
4* = 5* L2
5* = 4.33M-5M
Level 4
3* = 4* L3
4* = 5* L3
5* = 5M-8M
Level 5
3* = 4* L4
4* = 5* L4
5* = 6.5M-9M
Level 6
3* = 4* L5
4* = 5* L5
5* = 13M+
...
Level 21
5* = 107M+
From these, they will be updated soon as I received information from people at those levels. These will help you spot a fast trainer in the transfer market, thank you.
Newbies' information
The term "fast trainer" is used for a player, with the age ranged from 18-23 year old, who is more expensive than normal 5* players in the market. They will usually gaining skill points in almost every matches, and can be trained easily. When you do power training, he will gain more skill progresses than the other youngsters in your team.