When you use algorithmic optimization, setting a maximum bid doesn't mean a placement is going to reach that bid. Assuming a placement is meeting the desired goals, the bid step, learn budget, and campaign duration will impact the bid price.
Remember that smaller maximum bids, give the optimizer fewer options when making changes to the default bid to win inventory.
Consider what inventory you want a campaign to win: browse in the inventory screen some domains that you would be interested in winning impressions at and check their price. Set your max bid at $1 - $2 higher than this.
eCPC, eCPA, or eCPCV
For these goal types, choose a maximum bid based on the campaign goal as well as a reasonable estimation of "best case" CTR, conversion rate or completion rate.