Sorry if this has already been said (didn't have time to read the whole thing), I've done it both ways but for me it all comes down to this:
Can you buy them in bulk, or are you paying pet store prices? You'll always lose if you're paying someone 10 times what they're worth.
How much do you value your time? You can probably pay someone $8 / hr to breed mice. If your time is worth more than that, it's probably better to purchase them (bulk pricing of course.)
So, if your time is valuable or you have very little of it, I recommend buying them from someone who can do it cheaper than you, ONLY IF YOU GET BULK PRICING.
Lastly, how much would you enjoy breeding them? I actually like doing it, but decided it wasn't worth my time.