The answer to what a 1912 Irving Lewis (Boston-NL) tobacco card is worth is quite simply; "it depends". I'll explain why this is in greater detail below.
An original 1912 Lewis tobacco card is potentially worth a large amount of money in today's market. It's not possible to give accurate figures as it's rare that one is actually sold. An original 1912 Lewis tobacco card, however, is estimated to be worth anywhere between $4,000 and $7,000.
The issue with valuing a tobacco card is that there have been many reprints of the cards over the years. Reprinted tobacco cards do not hold the value of originals, and even though a 1912 Lewis tobacco card may state a value of $2,000, this could also be a reprint. A reprinted 1912 Lewis tobacco card would be worth around $8 on auction websites such as eBay.
Without being able to confirm whether a 1912 Lewis tobacco card is original or not, it's not possible to confirm its worth. It's always worth trying to verify whether the card is an original or not, as this could lead to a small fortune if it is.