... there's a propane-based or natural-based or oil-based system. I'd at least contact those folks (among others) for info. Maybe they have cost figures for you too. DO tell them you're over an UNinsulated crawlspace too. And ask what you/they should do as part of the install with that in mind.
If it's easy enough to rip up the sub-floor, I'd consider that. You could re-use the existing sub-flooring after you insulate. (If you do this, number or letter each sheet BEFORE you take them out.) IF you replace the old flooring w/new flooring, use the old sheets as a template. Remember, the old OR the new sheets won't fit EXACTLY in place because they're a little bit smaller, in size, once they're cut because of the sawblade. That's OK in my book. Make sure you install at 90 degrees to the joists and subsequent layers, that are added ON TOP of the sub-floor, are at 90 degrees to each other. Use screws to attach the decking to the joists. Install the insulation with the Kraft paper facing UP; not down. And before you lay in the insulation remember to staple up some string on the UNDERSIDE of the joists to create a 'net' for the insulation. Use nylon string because it won't rot. Space it every 10", give or take. (You don't want it to sag downwards ...) Use long staples too.