For a standard bacterial transformation experiment you'll need : An incubator, a pcr machine, taq, primers, pcr master mix, a centrifuge, petri plates, LB broth, agar, ecoli, a gel box. Honestly, half of this you could get as cast-offs from your local university for free. Universities are all about supporting K-12 science, you just have to ask.

Hardware is going to run you about 200-300 bucks. The real expense here is the used PCR machine. The incubator, and the centrifuge can be home-made or repurposed (ie a hand crank centrifuge, and a chicken egg incubator).

The rest of the cost is just reagents. The two big things you need to make yourself are Taq, and ecoli competent cells. Both of these are fun experiments in themselves are can be made for 50-100 bucks (resulting in years worth of reagents).

After that it's just buying primers (10-20 bucks a pop) and refills on reagents like agarose and LB which are cheap.

DIY lab doesn't have to be expensive if you are willing to sacrifice a little bit of time.