is it worth upgrading from rear v brake to just disk on back wheel of my mountain bike?

The cost will vary. First, is your frame disc brake ready? If not, you simply can't use disc brakes.

Next, is your hub/wheel disc ready? If not, you will need a new hub or wheel.

Just adding a disc brake to the rear isn't really a good idea. The feel between the front and rear will be very different.

As others have said, the front provides most of the stopping power. V-brakes can get the job done, but often V-brakes are not adjusted properly. As the pads wear, they need a little adjusting. Pads should be close to the rim to provide good stopping power. If they rub, the wheel(s) need truing.

Replace worn pads, keep brakes adjusted, wheels trued and V-brakes will do a good job.

Any decent v-brake has more than enough power to lock the wheel, so if yours aren't working there is something wrong. The place to upgrade brakes is on the front wheel where you get a lot more braking power. Unless your wheels are disc ready, you get a lot more bang for the buck upgrading your v-brakes, that is what I did. For the price of a single mechanical disc brake caliper, I was able to upgrade both my brakes to Shimano XT v-brakes. These are light and powerful, easily as good as any mechanical disc brake. If you hardly use your front brake, you need to start practicing. I use my rear brake for controling my speed. When I really need to stop, my front brake is needed because under any hard braking the rear wheel locks up. Even a set of new brake pads and cables could make a big difference to your brakes. Before you start buying expensive parts, make sure the ones you have now are working properly.

The front does the best and most braking, but the old coaster brakes were only on the rear.

If you loose traction, your backs are useless. Controlling speed downhill, but for stopping

they are useless.

Unless you have hubs that accept a rear disc, it is too costly to add disk. But V brakes are

fine brakes, I am not switching anytime soon.

Aeromax Alloy Wheelset Road Bike Comp 700c Wheels roll great, and my set came damn near perfectly trued... the bearings were packed a little tight, but it was simple to correct that. I do a self created roll test on new wheels when I purchase them.. I use the same tire for every test for consistency, and these wheels roll better than the 1000$ Eastons I purchased last summer... Im blown away by the price and quality of this wheel... they are comparable to your 500 to 1000 dollar wheels... Mine have stayed trued for almost 4 months now. these are a no brainer!

Unless you're getting good Hydraulic Disk brakes, it's not worth it. Mechanical disk brakes aren't much better that good linear pull brakes and weigh and cost much more.

Your front brake provides 75% of your stopping power learn to use it.