Excel cell auto change or appear data next to it ( or itself)?

First I believe you meant to add the cost in B1, not A2.

Here's what I suggest -

1. In an Excel workbook, let Sheet 1 be your master list that contains the 3000 product number & cost in column A & B

2. Copy your downloaded data to Sheet 2 - assuming the product number is in A column, then you insert a column in front of B, now enter the following formula in B2 -

=VLOOKUP(A2,Sheet1!A:B,2,0)

3. Copy this formula all the way down, now you should have all the cost in B column

4. You can create a Macro for this process (insert column > type in formula > copy all the way down) so you don't have to repeat the steps every time after the download

As someone has said, you can wrap text, or even merge a couple of cells together. To wrap the text, either right button of mouse, format cells, alignment, wrap text or, if you have already typed it, click in the cell, then click in the formula bar where you can see it and where you want the break in lines use Alt Enter to force the words onto next line within same cell. Excel is not really designed for long descriptions and as you yourself say it is 'comments' perhaps you should consider using a comment instead of normal cell (right button of mouse insert comment). The comment is then in a separate box which you can move away from the cell in case it covers other data in other columns, and right button of mouse to show or edit comment. However, as you appear to be using your spreadsheet purely for text, I would advise using a table in Word instead which is more used to text. In that case assuming 2.5 cm margins all the way round and you are working in A4 landscape your first column could be 2 cm wide to accommodate name and the second column 22cm wide to accommodate the comments. You can always cut and paste rather than retying if you want to try this. It would then automatically go into Word as a table and you just need to change the widths. Another advantage of using Word is, as you type in one cell, when it gets to the end margin of that particular cell the text would automatically wrap to the next line.

I don't understand your problem very well, but there's a formula called VLOOKUP that's very used full. It goes like this;
eg;
=vlookup(A1,the table,column no,false)

the table should have your product as the first column ans the column no is no of the column in which the needed info is in.

hope it helps