purchase order quantities and cost

Hi:

Question: Since we don't have the ability to do a purchase order, then we just count the inventory and add the quantities purchased to the product file?

and how about a cost change--we just changed the cost in the product file?

What happens to the existing inventory purchased at the old price? will that reflect the new price?

how do you suggest we handle this?

thank you so much, if you can assist us with this.