I assume there will be free updates, some including content/ challenges as seen with Age 2 DE (new free challenge missions for example) but I wouldnt expect too much.
As for DLC price, I would argue regarding the civs it brings and content added.
The base game has 8 civs and 4 campaigns for 60€ here.
If a DLC is 2 new civs with 1 campaign each (so 2 campaigns) I would expect the price to be around 1/4 of the base game ca. Maybe a bit more because 2 campaigns would be half of the OG content. So lets say 16-18€.
Another (although due to asymmetric civs more unlikely) way would be a mini DLC: 1 civ with only 1 Scenario mission - that would be like 5€. But I do not believe this will happen.
1 civ with a campaign maybe, then it would be around 10€ /1/8 of civ content, but 1/4 of campaigns)