Yes beef tenderloin is the same thing and same cut of meat as filet mignon.
Filet Mignon is the French name for beef tenderloin.
The cost of beef tenderloin can vary from store to store and location to location.
But at Costco here the beef tenderloin is $14.99 per lb.
At Trader Joe's the Beef Tenderloin is $17.99 per lb and at Whole Foods Market it's $29.99 per lb.
However that is here but in your location it could be different in cost and it's different in cost per lb at different stores.
But expect to pay between $14.99 per lb to as much as $30.00 to $35.00 per lb of beef tenderloin depending on where you buy it.
Beef tenderloin is not cheap but it's worth it if you can afford it.