I'm checking the item ledger entries and I see the lot number:
But where can I see the price of that lot?
For example the first line, a Purchase for lot no. 1234-3 where can I check the cost of that purcharsed lot?
Also for sales the sales price... for posted assembly it also should have a price
Where can I check it?
Thanks in advance

Standard Item Ledger Entries page in Business Central contains columns with cost and sales amounts (for sales entries).
If you can't see these columns, this means that the page you are viewing has been customized and the fields are hidden. You can try do return them to the page using the Personalize action in top right corner.
This link has a detailed manual and few nice videos: https://learn.microsoft.com/en-us/dynamics365/business-central/ui-personalization-user
But if you don't see the "Personalize" action in the page, likely you don't have permissions and have to contact someone who is supporting the system and ask them to do this for you.