$300 for a full finished commission would be on the low side I would say. Jango is not as in depth and time consuming as a ROTJ paint job but there is still many hours worth of work to clean, prep, paint and weather a helmet. You will be about $200 in on just the raw helmet. I would expect a paint commission to be at least $150 on the low side. If I were doing it I would probably be quite a bit higher than that (not that I have time to do commissions).
Just to give an idea.....if it is a nice cold cast helmet you will spend several hours sanding and polishing the whole thing to a shine. Then you have to mask off the areas and prime and paint. Each paint color needs to be added in smooth thin layers with dry time between each and it is best to have some cure time between colors .....the two recommended colors don't always play well with each other..... Then smoothing out the paint. Attaching ears and RF. Pin striping and weathering. And lets not forget installing the visor. So as you can see there is work to be done to get one finished right. And the built in wait times add up.
Hope that helps.