Ressearching that ammount of scrolls and magical items is going to cost
a lot, not to mention that the components you need might be a bit rare.
The costs only rise if the wizard is going to make a lot of scrolls of
the same spell. What if you need a whisker of a hell hound for the inkt
of the scroll of fireball? Getting hell hounds is not easy, is it? Why
else do you think magical items are so rare?

When I take a look at the published adventures of Birthright (granted
that are not many) the main villains do not have access to huge ammounts
of scrolls. Even when they are working for a mage like the Serpent, the
villain gets only one scroll to make the plot possible at all (Cities of
Sun boxed set). So the component thing probably is a big limiting factor
in the production of scrolls. Though in this case the Serpent might have
feared to be double crossed by his agent.

As a side note, do you use priest battle spells? The mass sanctuary
spell is a very good defense against a wizard. The wizard needs to make
a save to even attack this protected unit. A very easy way to approach
the unit you want. Ask my players they saw a trap being destroyed
because there archers could not take advantage of the akward situation
the enemy was placed in due to the trap. Since that day the temple has
become very important for the ruler.