Do you mean the Anomaly scan at 20 weeks? If you weren't going to get any other scan in the entire pregnancy you should definitely be getting this one.
To be honest if you only had one scan for your entire pregnancy it would be this one. At this point all the organs are well formed and can be thoroughly checked, this is the scan that could pick up any defects that would possibly require any in utero surgery or treatment ready at birth so basically its a scan that could save your baby's life (vs the 12 week scan which is more or less screening for defects which might lead you to consider termination)
You want this scan to be done thoroughly and properly by well trained experienced senior radiologist/sonographer. Some of the not so good ones tend to spend all their time looking to determine the baby's gender, or taking 3D pics which although lovely, really isn't the point of the scan. They should spend at least 20 minutes checking all the chambers of the heart, the optic nerve, the palate, the blood flow between organs etc etc.
If you Insurance doesn't cover then pay, but I'm very surprised it doesn't cover, is it covering routine scans at OBGYn consults, they are much less necessary. Mine has always covered all scans.