I bought some 4' long aluminum diamond patterned ramps from Inudstrial Metal Supply in San Diego for about $80 total and have used them before semisucesfully in mud and snow.
and by the way, these are 31" bfg mud-terrains looking more like cheese wheels
and just in case you are thinking what am I doing in the middle of some field tearing up the land? - we were following a dirt road and we got to a huge washed out rut that we couldn't cross, so we had to turn around, the road wasn't wide enough so I decided to drive out to the side of the road onto a plain looking green field to make a u-turn. well as soon as I entered the green field the truck just sank right into the super-soft mud. was one of these moments that reminds you that not everything is the way it looks...
now back to the topic. these boards are not anywhere strong enough for bridging though, but they double as my motorcycle ramps (if I can find a nice hill, cause they are somewhat short for the tall bed of my truck) and sometimes even as tortilla heating pan, although I heard aluminum is not good for your health
I wish I could remember who suggested these to me, but I can't. Anyway the cheapest idea, that works very well and is much safer than any metal sand ladder or ramp is as follows:
Get a used snowmobile track (usually free or close to it) cut the track in half and you have 2 steel reinforced rubber mats with traction ribs or paddles on one side and raised rubber knobs on the other. They measure approx 12" x 4'. For storage you can roll them up, or lay them flat. They don't work very well for heating tortillas, but they also won't damage your vehicle and (more importantly) won't hurt bystanders.
sand ladders work if used properly, i have a set that i made and they have caribeaners on pieces of cable that i attach to rock rails on the truck so they wont fling out when u really gas it, realistically the ladders are supposed to be a pre laid trail that you cross slowly, like a bridge.