How much would it cost to change my scooter folding handle bars to a solid handle bar. I have the nanrobot D4+

You would need to go the shop and see if that's possible. If it is, select from the in stock items or order your own then bring it in to be changed. The charges will be explained to you in the store.

Around $100 for the bar and adapter if needed and labor

I agree totally with bro. Sean S. You have to stop in the shop 1st. Adjustments that are made are unique to each scooter. Short, Tall, Weight, wide, thin, thick etc.. So it's best to stop in, that way you get exactly 💯 what you want. Most importantly safe & sound to ride. Good luck with your update bro G Villanueva👍🏾👊🏾