Time, you forgot the time. So 2 hours a week cleaning feeding ect. 104 hours of labour maybe more. Say ten animals a clutch, so spiders, say you get lucky and have 7 spiders at 300$ each and can sell them all reasonably quickly. The costs of power and food for the breeders for a year is 200$ The normals you have are basically a loss you make nothing on them by the time you can sell them at 40 bucks they have cost you close to that. 1400 - 200=1200 ignoring the cost of the initial investment and materials you are making 11.54 an hour. That is conservative if you expense 30% capital a year it is now 9.81 an hour, that is basically minimum wage where I live. That is just the spider and normal and a capital of 600.
A friend and breeder where I live took 9 years to break even and that is not counting an hourly wage at all. The next kicker is sooner or later if you keep expending you will need more time than you have, and that means expand again and give up your day job or stay small and not really pay your self. It can be done but not as easily as it seems.