Breed what you like and you will always be happy with what you produce.
The market is supply and demand. When the supply increases the demand and price will decrease.
The future is combos. The days of one co-dom to 10 normals doesnt make sense today like it did when a mojave(for example) was $4,000.
If you are in it strictly for investment reasons then you probably wont be very happy with the results. There are much safer ways to invest your $ that dont require cleaning poop and raising rodents.