I am looking to have zen-cart store & calculate my product costs so I can calculate my inventory value & therefore profit each month & most importantly, at the end of the year for tax.

I have found these plugins:
https://www.zen-cart.com/downloads.php?do=file&id=1444
https://www.zen-cart.com/downloads.php?do=file&id=175
They will not work for me as they assume that each product only has one cost. If there is a price change part way through the year, then they cannot deal with it (if I am wrong, please correct me).

Example:
March I buy 10 units at $1.00 each
June I buy 10 units at $1.20 each.
December I still have 20 units, at an average cost of $1.10
Most of my products will have a different price each time I purchase them.

My accounting package does not have a great inventory system, so I think that zc will deal better with the few thousand inventory lines I have.

I believe that I can do it by adding a couple of tables:
table: purchases
fields:
purchases_id
date
supplier
invoice number
This would give me an audit trail back to the invoice

table: purchases_products
fields:
purchases_id
product_id
quantity
price
This would give the $ spent on each individual product.

Add a page similar to the current products admin page & when I get a new shipment, I can add the number of units & cost of each product, click 'update' & store the value at the same time I increase my stock numbers.

I am not sure what my SQL would look like to get the LIFO data for the current values (might have a look & see if Mr taxman says WAC is ok, it feels easier), but I know it will be possible if the data is there.

So, my questions:
1/ Has someone else already invented this & I am an idiot for not finding it?
2/ Has no-one invented it for a very good reason & I am an idiot for not doing the same way in X very simple way
3/ Is this actually a reasonable idea which could be implemented without a massive amount of anguish?
4/ How do other people value their inventory at year end (remember 2000 items & regular price changes)

Thanks for your help.

Pat