It's not just about what you think extra bongos are worth to you. A bunhc of money was spent developing the game, and the bongos cost a certain amount of money to produce. They're packing one set of bongos with the game and selling it for the standard price of $50.
They probably can't really afford to sell the extra bongos cheaper than $30, otherwise, they probably would.
Anyways, the game's a lot of fun but there needs to be an expansion. I want more songs. Plus it is kind of dissappointing that none of the songs are performed by the original artists.
I guess Nintendo was trying to keep the licensing costs down. If you check the manual, all of the songs are licensed from EMI/Virgin. A larger soundtrack obviously would have cost more money, and probably be from multiple publishers, further adding to cost.
Still, there should have been a lot more Nintendo game songs. Really there are only three in the game. I can think of a whole ton of Nintendo game songs that they could have had on Donkey Konga. The DKC soundtrack would have been a good place to start.