It is quite commonly acknowledged that using BA Miles to book economy seats is not going to result in a "good deal" unless it is a last minute thing where only full fare Y tickets would cost a heck of a lot of money.

It is a common theme in Frequent Flyer Land that the best "value" is using the miles to book seats in Business of First class, or using them to upgrade from Premium Economy to Business class etc...

So, I am afraid, if you are looking to use them for economy travel, the fees will be high versus what you could pay outright for a seat to begin with.

It is just a different way that the program operates versus, for example, an American based FF program.