the actual development of GBA games in cheap when compared to todays multi-million dollar budgeted mega games.
However, the cost of manufacturing is very high. Therefore, developing for GBA is a high risk proposition. Manufactuers have to be sure they have a hit on their hands, and have to properly gage how many carts yto make at one time. Make to many and you end up with a glt of games that just cost you 15 dollars a pop to put together, and aren't ever going to sell. Make too few, and you will sell out, and by the time you manufacturer, the game will no longer be marketable.