See attached - I charge based on time initially worked. I don't think this set up would take longer than 10 minutes, honestly, and it wouldn't require any maintenance.
If you wanted artistic enhancement or sculpting, etc, of the area they arrive in - that'd be extra, but it'd also be quick. I have an affinity with illusory magics as well as sculpting of rock. The former would need to be maintained about once every few weeks, the latter would be stable for as long as you'd like it.