Again, 5,000 btc marketbuy gets you there and probably 10,000-20,000 btc will hold you there...
The way to think of it conceptually is with the price elasticity on buy(sell) pressure.
This is defined as the increase_of_marketcap : increase_in_investment
In the past, I have estimated that with cryptos, this ranges from 4-10 over longer periods. Eg. CryptoKingdom has now marketcap of 700,000 XMR and perhaps about 70,000 have been invested to it, this makes the price elasticity ratio 10.
If the same ratio applies with the escape of capital from BTC to XMR, without change in the total cake, they meet halfway as follows (figures in $100M precision)
Start: BTC_mktcap: 3,800M, XMR_mktcap: 0M.
End: BTC_mktcap: 1,900M, XMR_mktcap: 1,900M
Capital flight from BTC: (3,800M - 1,900M) / 10 = 190M.
Number of BTC exchanged for XMR as "buy/sell pressure": ~1 million (because the USD/BTC halved from 250 to 125 during the time).
Math is different if new money makes up for the bulk of the buy pressure. Still, I believe the 4-10 ratio holds for fiat/XMR as it did for fiat/BTC.
In this case, about 400M-1,000M USD is needed to reach BTC marketcap.
Many conditions apply, time is ignored, etc. but you get the basic point.