I'd recommend keeping a culture of d. melanogaster on hand; they produce much quicker than hydei do. I also recommend having a backup culture running. Again, the cost to make your own cultures is about 10-15% of the average cost of store bought flies... If you make 2 cultures of melanogasters every week, you will be spending at most 4$/week, but will always have flies. I either freeze off excess or sprinkle them in the yard for my army of jumping spiders.

There is a little bit you should know about making your own cultures though should you choose that route... The initial investment is a little high however; you're going to save ALOT! You'll probably spend around $100 dollars (equivalent cost of 10 store bought cultures) for materials to make 65-80 cultures. Here is what you'll need:

1) Cups and lids (I buy in bulk from online vendors. PM me if you want to know which one I use).

2) Media (you can buy pre-mixed if you like, I think it's cheaper to make your own). This consists of the following should you choose to make your own:
a) Potato Flakes
b) Sugar (I use pure cane granulated)
c) Brewer's Yeast (I buy from vendor)
d) Methyl Paraben (I buy from vendor)

3) Baker's Yeast (not required but many old school dart froggers swear by it)

4) Excelsior or coffee filters (I don't use filters, I have bad luck with them getting soggy plus excelsior seems to hold the media in place better.

I hope this information is useful... It's a pain in the rear to type all this on an iPhone...

-Christian