Budget a science fair project.
A science fair budget has a fixed setup cost and a per-trial cost, and the per-trial component is what usually breaks the estimate — thirty trials at a modest consumable cost each can exceed the equipment. A contingency of ten to twenty percent is not padding: failed runs that must be repeated are the norm in a first experiment, and a budget with no reserve forces a reduced trial count.
Cost build-up
Subtotal = materials + trials x consumable cost per trial + display and printing
With contingency
Total = subtotal x (1 + contingency); cost per trial = total / trials
At least three per condition to show variability, and more where results are noisy. Judges consistently reward evidence of replication over a single dramatic result.
Display materials and printing, which do not affect the science. Cutting trial numbers to save consumable cost weakens the project where it matters most.