There's a total of ~1,300,000 BTC available on all major exchanges according to viewbase.com with sell offers spanning from 42000 to 100000 and probably higher in the order book. So I'd guess if we pick average price of 70000 per BTC you'll have to pay ~$100 billion. But it shouldn't be as cheap as that because markets will react to such rapid buying and adjust their offers up in price. Or you could start buying... Source: about 2 years ago

I am actually working on the whitepaper right now. Which I will be submitting my patent idea very shortly. To secure my idea on https://originstamp.com. Source: about 2 years ago

P.S. You should use OriginStamp or something similar to put the hash of the cafe transaction ID (a hash of a hash, I suppose) on a blockchain and tell us about it so that you can later prove that you did have a specific target transaction in mind. Otherwise, you could unfairly "change your mind" about the target transaction later. Source: over 2 years ago

2) To prove that your video or photo were collected no later than a certain date, you can hash the file and then post it to a blockchain, such as BTC or BCH. This service does that for you. I or others could help you do the technical parts. It shouldn't be too hard. Source: almost 3 years ago