Won't it get expensive adding staff to my business after a while?

Short answer is no - all addons are for all your businesses, not limited to a single business.

For example:

Say you have 10 businesses, you would add 9 more business instances at $5 each this will be $45/month

Then if you add a $5/5 pack each of those businesses would get 5 extra users.
So you would have 50 staff for $50