Wouldn't it be foolish for microsoft to give them a discount?
Like you could discount say a 10B investment down to 6B or w/e the actual costs are but now your share of OpenAI is based on 6B. Seems better to me to give OpenAI a "10B" investment that internally only costs 6B so your share of OpenAI is based on 10B.
Plus OpenAI probably likes the 10B investment more as it raises their market cap.