$300 seems about right. 6 litres of proper, full synth motorcycle oil and oil filter plus, if they do it properly, 1-2 hours to do it.
I used to use BellRay EXS oil in mine. Cost about $120 for 4 litres (so $180 for 6 litres); filter around $20. $100 for their time and expertise, the confidence of any updates being loaded, checks for error codes, recall notices and service bulletins and peace of mind seems like a bargain to me. Cutting corners on that and risking your warranty to save a few bucks makes no sense at all.
16,000km until your next service - this first service therefore costing less than 2c per km. Even if you do a mid interval oil change the cost is minimal and well worth doing it properly. I'll never understand why people so concerned about spending money on servicing or tyres or fuel buy top end motorcycles, especially something out of the ordinary like a Rocket.