How many dollars of BTC purchases would it take to go to $10,000?
It's impossible to calculate. The order books listed on exchanges are fluid and only give you a glimpse of overall market liquidity. As the price rises, more coins will enter the order books and it'll take more money to keep pushing price up.
One could create a function to figure out an approximation. For instance, you know that once it reaches $4000, then it would be more expensive to buy it. Same thing with each dollar it climbs. You could say it would cost 4 million to reach $10,000 a BTC, or you could say 50 million. One is more accurate.
it is easy to make as long as the exchanges have an API call function that has the ability to give you the orders in the sell order book. then it is just a matter of making the call and then adding the values, you may not even have to multiply anything since many of them do report it in both BTC and USD.
additionally, for the lazies, you can just open the exchange webpage and look at the charts, most of them have an order book chart which shows a V like chart representing the total amounts inside the order books.
but since there is no point in doing something like that, nobody is going to do it ever. you have to do it yourself if you want to know that value. as @squatter mentioned the market is dynamic, when the price starts rising that is the time when a lot of orders will start being placed. not to mention that many will sell at the market price without even placing an order so you don't see them in the order book now.