So, in simple excel terms:
Created a new column for prices that will be copied and then the values pasted over the existing prices in tht column I put the following code:
Code:
=IFERROR(VLOOKUP({cell with original model#},{entire table of new data},{column number of new price},FALSE),{cell with old price})
Where:
{cell with original model#} for example would be replaced in whole with a cell number such as A2.
{entire table of new data} would be in a format such as: Z$2::AC$5000 to include all of the new data that has 4 columns with 4999 items in. The new list and model number (or the value being used to compare) is in the far left column.
The column number is only a number not a cell reference. The number relates to the columns as a part of the table selected above. So, the column to the right of the first column would be column 2, the last column (right side) would be column 4.
The value of FALSE is used to return the associated value of the existing new entry, if the new data does not include the searched item, then an error is returned and through the above code, the default price is used; however, an alternative may be desired such as to disable the product or to delete it from the database (though I don't out-right recommend deletion as it will have an effect on historical lookup information).
Essentially then, the new price column would be copied and using paste special the values to replace the originals, delete any new/unnecessary information, then upload the prices to reflect the new prices of items and existing prices of items not in the new list.
This process does not specifically identify new items or the deletion of items, but a similar approach can be taken to identify those and to make other associated changes.