I don't think lowering the cost of items bought with BP is necessary.
Let's examine how you earn TC. In most Battle Tower matches, you will be gaining 2 TC per match. From my experience, these battles normally take about a week to finish if you have an active opponent and ref. You also gain 1 TC per battle completed in the Battle Hall. Assuming that it takes two weeks and seven battles for your Hall Challenge to be completed, which is kind of in the middle, you will have gained 19 TC in two weeks if you have active refs and opponents.
Now let's examine how you earn BP. Let's say that your Subway Challenge takes two weeks to finish (a fair estimate) and you gain 6 BP by losing to Ingo. In addition, we assume the same statistics about the Battle Hall. In two weeks, you earn 13 BP, once again assuming that you have active refs, opponents, and a strong team. Remember, this is assuming that you lose to Ingo every single time!
So, looking at this, it seems as if BP is much harder to obtain than normal battles, especially since the Battle facilities often require trained Pokemon. However, you must also remember that there are more Battle facilities that will be coming. The Factory is currently in its
Preparations stage, a rough idea of the Arcade has been developed, and the Castle is [still] making movesets. Even if only one of these facilities end up getting implemented, your net BP gain will be approximately equal to your net TC gain.
In summary, BP is currently pretty scarce, given that there are only two sources to obtain them from. In contrast, there are four sources you can gain TC from (Hall and 3 battles), and this isn't counting Amulet Coin or other such shenanigans. As the other battle facilities get developed, BP will be much more accessible, which is pretty much why I'm against this.