Twenty Percent - How does one afford it?

I was finally approved for a power wheelchair last year and ever since I have been struggling to actually get one that fits me, NuMotion, and now one that I can afford, Fletcher's Medical. 

My first order was with NuMotion and when they delivered the wheelchair after a hefty copay, I was surprised by its size. After careful review and my wheelchair therapist seeing video of the wheelchair, she concurred that the wheelchair was too big for me. I was told to return it and ask for a copay refund. I am not pointing fingers but my therapist believes they delivered a possible return to me in hopes that I would accept it and allow them to install cushions so that it would fit me. 

I was then told to go to Fletcher's Medical and get a new evaluation due to the fact that too much time had elapsed between the last review. I just received the invoice and my share, 20 percent, was the cost for a SmartDrive MX2+. Astounding. 

How do insurance companies come up with these balloon "Allowable" amounts?  

What am I missing in getting a power wheelchair at a reasonable copay? I am used to copays for manual wheelchairs and now these copays are almost four times that amount. Help!