"How much" as a question is quite vague. Some hot water heaters are comprised of copper only. So this now depends on the size of the hot water heater. Some hot water heaters have no copper whatsoever.
Yes, the tankless water heaters do cost more than the normal water heaters because tankless water heaters are designed to supply hot water only when needed unlike normal ones.
American Water Heaters.
American Water Heaters
Electric water heaters have not carbon monoxide.
You could drain the line, sandcloth flux and solder it. Otherwise, see if you can get a compression fitting that would fit the copper tubing
You can buy water heaters from any home hardware store.
GE hot water heaters are made by Rheem.
Rheem provide a series of high efficiency gas water heaters. Several different companies also provide tank-less water heaters, which are very efficient, as well as solar water heaters.
"Rheem does manufacture both gas and electric water heaters. Rheem currently offers 18 different varieties of gas water heaters and 9 different varieties of electric water heaters. Both types of Rheem water heaters are known for their rigorous standards, reliability, and efficiency."
Water heaters can range from 1 unit to 10 units.
Some of the water heaters uses magnets. Magnets softens the water.