Can I add a transformer to derive a neutral?

[petersonra]

I think it will cost more than $2-300 just in labor to add the 120 V xfmr for the receptacle, unless your labor is free.

I bet the trip to the supply house just to get the parts costs more then $200.

 

[LarryFine]

Give them the price for just powering the sign, and then the additional cost of providing a receptacle.

As long as the customer is willing to pay the cost for what they want, get the job and do the work.