Materials would cost me about $20 for the wood (not too sure about 1" MDF though and it wouldn't be needed for a 3.5 cube sealed enclosure), $2 for the glue, ~$3 wear and tear on tools, and then about $10-30 on the paint depending on what type of paint you wanted.
That totals to about $45 +$10 in just materials. Then I would consider what type of finish that was requested to decide how much I would take from the transaction. If it was a basic spray can finish, I would charge about $40 for my time and effort. If it was a gloss finish of some sort or something that required a ton of sanding and finishing, the price could go up to $100 for my time as it can be a lot of work to get a nice finish.
So for somebody local, it could come out to only about $85 for a basic job or up to about $160 for a nicer finish.
That's the process I would go through when deciding what to charge. I will only do local work though so I would have no idea what to expect from shipping.