I just paid the 4,99 USD (= 3,98 €) for the complete BFT 12-mission-campaign.
Of course I would prefer the campaign to be free, but if my money helps making further development possible, I am gladly willing to pay a few bucks. In my country, you pay 2-3 times the amount of money to watch a movie in a cinema.
Just look at this thread. Sabre and his team started work on these missions in the end of 2010 and kept updating them until now. For this dedication and the fun I already had with these great missions I personally see no problem at all with supporting them.