Do what I do, factor in the cost of Github hosting into the costs for your clients. The marginal cost for an extra repo on Github is $5/month on a business plan or $2 a month for the smaller plans. As part of the contract for each of my projects, I note that there is a $100 maintenance fee per year to keep the project in an active state, and that any project that becomes archived will have a $250 fee to reinstate as active. So every month I send a letter and invoice to all my clients who haven't had work on a specific project done in the past year to bill them for the maintenance fee or give them the option of archiving the project. I actually have a few clients who pay the fee every year even though I haven't done work on a project in several years, because it's worth it to them to have it available immediately if they need something done in the future.