The old promise of "You'll have many, many dollars later if you give me a few dollars now".
Most often seen as the Nigerian Prince/lottery scam, where you'll have $11.4 million if you pay a $300 bribe, oh wait, there'll also be a $600 filing fee, and - forgot to mention - the $1400 business license, and - we're getting close - $3000 check insurance. We'll be able to release the money quite soon, though!
Note how the numbers escalate. They ask more as you gain confidence. In the above scam, they troll you out of $300+600+1400+3000, but you would've never agreed to $5300 at the start! This is gaming on a human mental fallacy called
This is the belief that "Since you are already invested this far in, it's better to follow through "to protect the original investment" or some such gobbledygook.
Otherwise known as "In for a penny, in for a pound".
In your case, having sunk $200 and hearing good news, $450 seemed sensible. And now, $1200 is an amount you're actually thinking about.
Well, that "somewhat more than doubling every time" is no accident. Because sociologically, that is what humans are most vulnerable to, and the scammers have figured that out. (they aren't social scientists, any more than the pigeons who learned how to beat the Monty Hall problem; they just know what works.)
If you really want to, you can send along $1200; but you won't get your $6000. You'll get some cock-and-bull story, and a request for an additional $2500-3000.
Predictable like clockwork.