AIMster...I normally add about 2000 every 3/4 months..I have never thought about it changing the average cost...
I just tried adding 2000 at the present price..changing PC by 100 percent; 50 percent; 0 percent....in all three cases it changed my average cost by approx. 2 cents.....
It looks like if you have a high enough number of shares and do not add to much you can just leave PC alone..adding cash to buy shares will not change your buy/sell much..I run a spreadsheet so it is easy to check things like this...if someone wants to add cash they should check and see what it does to their buy/sell..