Yes, you can value each individual lot of your closing stock by cost price or market price. Even the same shares bought in different lots can be valued differently. If you use the method that produces the lowest value for each lot, you will reduce your profit for that year, shifting some of it to later years. You must however value your opening stock for the year the same as you valued your closing stock for the previous year.
You can also value using replacement price, but I don't see its relevance for shares.
