Things to keep in mind:
At first, most scammers start with a small amount - $350-$500 for the visa. Some scammers also ask for "hotel" money for their hotel "accommodations" in Moscow while they are filing for their visa with the US Embassy. Some scammers offer to pay for visa themselves (as a "good faith" gesture)
After the "visa" is received, the scammer asks for money to buy plane tickets - usually around $700-$1800.
Some scammers also offer to pay for the tickets themselves. Often they discover than the tickets cost more than they can afford, so they offer the victim to "split" the costs
Many scammers claim that they used up all the lifesavings, and then some, to pay for visa and part of the tickets, but it was not enough so they need just a "small amount" to help them out.
some scammers claim that they went ahead and entered a legal bounding contract with a travel agency and now they HAVE to come up with the necessary amount for the tickets other wise the travel agency will "put them in prison" [no, it is not true. I have not heard of any Russian travel agencies putting their clients in prison yet]
Very often the money request letters are supplemented with her most seductive pictures.