Free bees. Minus the cost of labor. A starte pack of bees typically can cost 165-180$
Yes. Sometimes it's easy sometimes its more difficult. It all depends on where they are living. Unwanted swarms are the easiest. When you have to gut part of the exterior of a house not so much. Cool part is normally the keeper just destroys and removes. The homeowner has to fix.
Edit: beek typically charges homeowners labor costs to remove. So free bees and cash.
Varroa mites are the biggest threat to bees. Like ick or velvet it can wipe out a hive if it's not treated. But there are meds that you can treat and should treat with routinely to avoid issues.