How would you price a 10-hour monthly web development retainer?

I'd start around $100 to $125 an hour, so $1,000 to $1,250 a month for the reserved 10 hours. That's a starting point, not a universal rate. Adjust it for your costs, the work involved and how much availability they expect.

Look back at the actual time your earlier projects took, including the work you didn't bill for. That's a better pricing reference than the original estimates.

Price the availability as well as the task list. Ten hours available during normal working hours is different from being expected to jump on requests immediately or be on call.

Put the expected response time in writing, along with how requests are submitted and what happens after the 10 hours are used. If you do overflow work, agree on the rate before starting it. A review after 60 to 90 days would give you a chance to adjust based on the actual workload.

The scope line matters as much as the price here. Homepage edits and routine maintenance can fit the retainer, but a small request can turn into a new feature pretty quickly.

Define what counts toward the hours, including SEO monitoring, admin work and figuring out what's needed. For anything bigger, give them a separate estimate rather than letting it quietly eat the whole month.

You built their custom Liquid theme, so your familiarity with it has value. Don't feel obliged to undercut your rate just because some requests sound like quick edits. Custom theme work and SEO call for different skills than basic content updates.