"I have a card from the T207 set of Lewis, Boston. It's the variation with no patch. I see where it originally had a $700 value, but what is it worth today?"
(Here's an image of the card...I'll identify it in a minute.)
The first thing that looks out of place is the part about how it "originally" had a $700 value. How did that information become available? The answer: "It was written on the back of the card":
First of all, there is no way an authentic T207 card is going to have any information on how the card became rare. Nor will it have any information on how much it was worth. In fact, the mention of "T-207" on the back gives away the fact that it's not real; the T207 designation wasn't even given to the set until the 1930s.
This is one of five cards that were reprinted and included in Hygrade hobby kits during the 1980s. Besides this card, there were three T206 cards (Wagner, Plank, "Magie") and a 1933 Goudey Napoleon LaJoie that were reprinted. They were even found with different-colored blocks on the back: the yellow one above, green or red. They were changed to reflect price increases as the 1980s went on.
An authentic T207 card will feature a writeup about the player and an advertisement of one of teh cigarette brands mentioned above. Any vintage card that has an explanation that it was rare...or has a price given on it...is not a real card.
Today, there are cards with manufactured scarcities (serially numbered, "1-of-1" cards, etc.), but they didn't play those games with cards that long ago.