In order to increase the strength points and attribute bonuses of a source weapon, one would need the source beast core or the source beast mineral vein in the source beast's body.
As the core of a source beast, the source beast core was naturally very expensive. Moreover, the source beast core was usually not found, which resulted in the price of this material being extremely high.
The scarcity of the price, coupled with the high value of the weapon, usually resulted in a very high price. This was one of the reasons why a source weapon was so expensive.