Bob,
For depreciation, once he comes up with a reasonable estimate of throughput including your prints, you simply pay him its value on a per print basis. So just a hypothetical example to see the structure of how this would be done: Capital cost $3600. Fully depreciate over 36 months. Monthly fixed charge rate for depreciation is $100. Between the two of you the machine is expected to have an average throughput of say 50 prints per month of "X" size. Depreciation is then $2 per print. Something like that. So you make 20 prints on it, you pay $40 for depreciation. Add-in what the accounting manager data gives you to derive an average cost of ink per print of X size, add on a good 20% for maintenance ink, and that should cover it. You are supplying the paper, so no charge for that. Now you've built-up what you could pay for cost recovery before some friendly value for overheads and his time, so you may wish to bump up a bit for that. Perhaps something along those lines takes you closer. Still some research to be done going that route.