I have an old cabin with block foundation, no basement but has very narrow crawl space.
Pipes do freeze.
Need to insulate foundation walls from inside. Electic and water are in crawl space. Don't want to tear up floors for access.
What is the cheapest and most efficient way to insulate?

It sounds like you must not have forced-air ducting under there. Generally even a well-insulated heating system will generate enough warmth to keep pipes from freezing, as long as the crawlspace is decently sealed. Also, if your floors are well insulated then your pipes are basically running in a cold zone and I'm not sure how much benefit you will get from insulating the walls without at least some source of warmth down there.

If the pipes tend to freeze in the same place every time, it might be better to address the specific issue. Anything from leaving a light bulb burning in the problem area, to a couple of those wire-wrap pipe warmers and insulating the plumbing itself might eliminate the problem.

I think the first thing I would do is to check for open vents, cracks, or other openings allowing in outside air. You don't want to totally seal the crawlspace air tight, which is pretty impossible to do anyway, but control the ventilation.

If you can seal the crawl space from outside air you can install a humidex system http://www.humidexhome.com/ which is a vent fan mounted in the crawl space on one end of the house and a fresh air supply vent on the other coming from the living area of the house. The fan runs to ventilate the crawl which helps control the build up of humidity as it does it pulls heated air down into the space warming it up slightly.
But like La marlowe said you need to address any areas that tend to freeze all the time as most likely you have a air leak where it blows on the pipes all the time.