If I bought 100 shares at $100 then 2 weeks later bought an additional 100 stocks at a $150, does the cost basis for my original stocks stay the same price of $100?
you might be confusing yourself. its the most basic math. if you buy an orange at $1 and buy another orange at $3, do you own 2 oranges at $1 each? the answer is obvious.
you'll just have 100 shares at $100 and 100 shares at $150. thats an average of $125/share.
calculate it like this: (how much you just bought * price /total amount owned) do this for all your purchases to find the weighted average.
for example: if i bought 68 shares at $3 then 892 shares at $3.5 then 100 shares at $4... i have a total of 1060 shares. calculation of the average price of these shares: (68/1060*$3) + (892/1060*$3.5) + (100/1060*$4) = $3.515/share
All 200 shares are worth whatever you can find somebody willing to buy them for. If you're talking about a highly liquid stock with plenty of buyers and sellers, that's usually somewhere close to the last traded price that you will see widely reported on financial websites. If you just purchased 100 shares at $150, immediately afterwards you would likely be able to sell all 200 for near that price if you so choose.
There is a separate concept called cost basis, which is what you paid for those shares. That remains $100 for the first 100 shares as $150 for the second 100 shares, regardless of what happens to the market share price.
If you are determining the cost basis of your stocks on sale, the answer might differ based on which shares are sold. In total, your total cost basis is the average cost of all your holdings, and your total gain is the average gain of all holdings that you sold.
If you take your example and then sell 100 shares at $125, whether you record a capital gain, loss, or neither depends on which tax lot you use. Most brokers now let you explicitly set whether you sell First in First Out, Last in First Out, or Average, or others. Example: TD Ameritrade tax lot options
First in First Out would have use your shares bought at 100 -> 125. Gain is +25 * 100
Last in First Out Would use the shares bought at 150 -> 125. Loss of -25 * 100
Average would use the average cost of all of your holdings. 125-125 -> 0 gain