Here's a very quick calculator. It lets you compare the total interest and fees paid between two scenarios. You can choose the initial balance and number of months to payoff (this assumes that you want to repay the debt in full by the time the promo period ends). For each scenario, you can choose the BT fee (in %) and promo interest rate (in %).

Using a scenario of $3500 balance, 24 month payoff, and comparison between 0% fee/4% APR and 4% fee/0% APR, the difference is de minimus, $148 in total interest/fees vs. $140.

https://dl.dropboxusercontent.com/u/...omparison.xlsx