Can you feed CP's blood meal in their traps, not as a fertilizer? You can feed some sundew many different insects, milk and blood worms and betta fish pellets. If you can feed them rehydrated blood meal, it would be an incredibly cheap food.
I'm not sure exactly what it is in their food that they need, I'll have to rewatch the video on carnivorous plants which mentioned a study done where they did many tests and fed them milk.
If you can't, knowing more about what they need, could help find the cheapest good source you can buy in bulk.
For now, I bought frozen blood worms, 4oz for $6 or so USD.