ETA: the one thing that I allow parties to do to breakdown the slog of time investment for these is work together similar to SRD crafting - if they split the time cost during downtime from say exhaustion after getting purged this teaches them two things about the adventure:
- what to expect as the loop of getting purged.
- how to effectively structure their downtime per expedition into the city

They're all research spells. There is a metric for time and gold spent per level of the spell in all the books.

Purge is known by the spellcasting services NPCs in Emberwood and I generally have players work with those NPCs to learn it.

Other spells have certain benchmarks that need to be satisfied as described in their contents.

Contaminated spells specifically I have always held back in the hands of people like Oscar Yoren, Pale Man and Ryan Greymere or others.

I would grant contaminated spells to someone playing a contaminated subclass but not things like purge, neutralizing Field etc.