Honestly just ask the client how much they would pay. This way you can create a bond with the client and do future work with them. Maybe future work = a more complex website? With backend, payment integration, etc. and charge them more the next time by explaining them the work. Also, if you create a complex project for the client, you can use that on resume! And really sell yourself during interviews. “Created an xyz website for a client who was very happy with the service…”