Find out what the average going rate is in your area. If you can't legally charge, tell them what the average rate is and that you would gladly accept a donation for removing their bees.
Most who perform removals, start charging a fee at some point due to the complexity of certain jobs.
This removal may be relatively easy or quite difficult. If you can legally charge and perform the task successfully, as you will be dealing with your own personal learning curve, you should get paid for your time, if you feel you should.
If your motive is to simply attain another colony for your apiary, it may be more valuable to you to get the job and not charge, but inevitably it is of course your decision to charge or not, in the end.
What is that colony worth to you? There lies the rub.
...JP