How does the IRS see this idea? It would be unfortunate to get a large fine for your hobby.
To figure out a charge you would best add up your costs, rather than look at others in the market. If your costs are too high in relation to the alternatives then best not to start. Don't forget depreciation on all the equipment used. Typically a residual value of zero after three years, or possibly five. Also, applicable health and safety rules (including documentation, if required) over chemical purchase, storage and disposal?
I'd suggest you should be charging a lot more, or zero. If you 'charge' zero make an accurate breakdown of costs (not including time, as this is not a business, right?), show it to the potential customers and suggest that the customers make a donation to cover your costs?