depends on your plans, with the player.
if you like to keep him for 3+ seasons, it could be economical better to buy player B even when you had to spend your money early which could be missing for anoter benefical deal.(i count with around 1,5k extra merch. np odea if this is right in such low areas)
When the player is a trainee, the merch income would increase with skill so it could be useful earlier.
If the player planned only as backup, it could be taken a bit longer.