@Jovi-Barinho
Hi there,
Goalkeepers are cheaper beause their real life market value is lower than that of midfielders and attackers.
For 90 - 99 rated players you will have to keep an eye out at the transferlist. If you have enough Club Funds, you will have to only pay 1 Bosscoin (or 5 if they are boosted). Using the scout it will cost you 15 Bosscoins, but it is much more difficult to scout 90 - 99 rated players.
You can buy 100+ rated players both from the scout and transferlist, but they are far more expensive and cost a lot of Club Funds. Bosscoins can also see a hefty increase if you buy them from the scout, or if you do not have the necessary Club Funds.
TLDR: You need to be really good at buying and selling players. That way you can get enough Club Funds to buy that player directly from the transferlist and avoid having to spend an extreme amount of Bosscoins.