Don't pay $50 for a single cylinder. Think I paid like $16 per cylinder in late 2014. Try Fleetpride, or Truckpro. Don't know what you have in your neighborhood.
My M923A1 uses Quickstart brand 22 ounce cylinder, Part # LP535, NSN 2910-00-646-9727. Quick Start products, 815-562-5414.
Use the ether assist sparingly, and 1 cylinder will last for a long time. Be sure you have hot batteries, and good clean connections for optimal performance in colder climates. If you have weak batteries, using ether will worsen your starting woes and vapor lock the motor, if you don't break any parts.
Get the motor spinning first when cold, then use the ether assist to get the motor to light off. Do NOT spray a lot of ether in the intake, and then crank the motor over. Doing so stresses your internal motor parts, and can/will damage your starter motor. Do NOT use ether to start an already warmed up motor. The expansion rate of ether is very high and using it to start a hot motor can lead to big problems.
The reason diesels use ether assist to start them in colder climates is this: Diesel fuel needs to be heated to around 575 deg. F before it will combust or detonate easily. In order to start a diesel when the ambient temp is below 32 deg. F, you have to crank the motor over several revolutions, smashing the air and fuel in the cylinders and "warming" the parts. Doing so places a large demand on your starter and batteries Add into the equation ice cold fuel, and some cold batteries, and starting a cold truck becomes quite the task.
Introducing ether into the air intake tract causes an motor to start faster because ether is extremely combustible at even 0 deg. F. You do not have to heat it at all to detonate it. It is extremely dangerous and flammable. The only downsides to using ether is that it causes a large amount of stress on your engine internals, and it wipes/burns the engine oil off of your cylinder walls when used.
Two of the best things that you can add to your truck, besides a new set of batteries are:
1) 24 volt waterproof float charger to keep your batteries topped off.
2) Electric block heater. Easily installed, and do not underestimate what a block heater can do when left plugged in overnight in subzero temps. I have block heaters on several of my machines, and can't say enough good about them. I prefer a block heater over a can of ether any day.