One set for a baking soda and acid reaction (HCO3 + H -> H2O + CO2) requires:
2 hydrogen atoms #60110E (1 Hole 17mm White Atom) 37 cents each in 2020
3 oxygen atoms #60200E (2 Hole 105 Degree 23mm Red Atom) 55 cents each in 2020
1 carbon atom #60400E (4 Hole tetrahedral 23mm Black Atom) 64 cents each in 2020
6 bonds #61013E (Molymod Double-Triple Bonds) 23 cents each in 2020
Get at least one set per student pair and several extra atoms of each type and 10 or so spare bonds.
For 15 sets plus spares you'll spend about $100 with tax and shipping (2020).
Use their molecule builder to find out what atoms and bonds you need for any molecule at https://www.indigoinstruments.com/3D-molecular-model-builder.php
Magnifiers (10X plastic geology loupe) at https://www.indigoinstruments.com/magnifiers/geology_loupes/10X-loupe-f…
I add a ribbon for students to hang it around their necks.