Selected Answer
There is a rough design in the attached workbook. Look at the Quotation tab. You can select the material and the thickness in B2:B3, and you can enter different dimensions in B4:B5. This is the formula that calculates the price in B.
=VLOOKUP(B3,Prices,MATCH(B2,Prices[#Headers])-1,FALSE)*B4*B5
It does so by referencing the pricelist in the worksheet "Prices". Note that the table in it is also named "Prices", and that Excel creates a named range of the table's name which, therefore, is also "Prices".
Pay attention to the cell formats in Prices!A3:A6 and Quotation!B4:B5. Observe that you enter, for example "2" but you see "2.00 mm" or "2.0 in.". If you enter "2.00 mm" the appearance will be the same but the functionality will be destroyed.
Also pay attention to the data validation lists for Quotation!B2 and B3. B2 has a manullay entered list (Mild Steel,Stainless Steel) and you must pay careful attention to spell the words exactly as they are spelled in the table's captions. B3 has a reference to the table (=INDIRECT("Prices[Thickness]")). That is more convenient because you can change items in the table and find the changes immediately reflected in the dropdown. But if you change the column caption in the table the link will be broken until you reflect the change in the validation specification as well.