Cograts on been responsible and able to plan expenses at you age ! Think the idea about the Bearded Dragon is good one. A new 40 breeder with dome and heat lamp will be within your planned expense, specially if you find a nice used one. Beardie's have come down in price recently (at least locally) and have seen nice colored 4-6 in. baby leatherbacks for around $50-60. Do start a Dubia colony ASAP if thinking about one.

Another option would be one of the lower priced Veil Chameleons. Again will need to get a screen enclosure but maybe get lucky and find one in good shape at Craiglist or similar. Study hard, take care of your Pets, and have fun !